Develop Reference

How to implement toString to convert a number to a string?

I was asked during an interview to implement toString() to convert a number into a string.
toString()
n => s
123 => "123"
Aside from:
converting the number by concatenating an empty string
123+""
using the native toString() function
(123).toString()
creating a new string
String(123)
How else could toString() be implemented in javascript?

You can use it as the property name of an object.
function toString(value) {
// Coerces value to a primitive string (or symbol)
var obj = {};
obj[value] = true;
return Object.getOwnPropertyNames(obj)[0];
}
console.log(toString(123)); // 123 -> "123"
console.log(toString(1.23)); // 1.23 -> "1.23"
console.log(toString(NaN)); // NaN -> "NaN"
console.log(Infinity); // Infinity -> "Infinity"
console.log(toString(-0)); // -0 -> "0"
console.log(toString(1e99)); // 1e99 -> "1e+99"
You can also use DOM attributes:
var obj = document.createElement('div');
obj.setAttribute('data-toString', value);
return obj.getAttribute('data-toString');
Or join an array
return [value].join();
And a big etcetera. There are lots of things which internally use the ToString abstract operation.

This works for integers. It takes the number modulo 10 and divides it by 10 repeatedly, then adds 48 to the digits and uses String.fromCharCode to get a string value of the digits, then joins everything.
function toString(n){
var minus = (n < 0
? "-"
: ""),
result = [];
n = Math.abs(n);
while(n > 0){
result.unshift(n % 10);
n = Math.floor(n / 10);
}
return minus + (result.map(function(d){
return String.fromCharCode(d + 48);
})
.join("") || "0");
}
console.log(toString(123123));
console.log(toString(999));
console.log(toString(0));
console.log(toString(-1));

The trick here is to consider a number as a series of digits. This is not an inherent property of numbers, since the base-10 representation that we use is quite arbitrary. But once a number is represented as a series of digits, it is quite easy to convert each digit individually to a string, and concatenate all such strings.
EDIT: As pointed out, this only takes integers into consideration (which is probably acceptable for an interview question).
var intToDigits = function(n) {
var highestPow = 1;
while (highestPow < n) highestPow *= 10;
var div = highestPow / 10;
// div is now the largest multiple of 10 smaller than n
var digits = [];
do {
var digit = Math.floor(n / div);
n = n - (digit * div);
div /= 10;
digits.push(digit);
} while (n > 0);
return digits;
};
var toString = function(n) {
var digitArr = intToDigits(n);
return digitArr.map(function(n) {
return "0123456789"[n];
}).join('');
};
Usage:
>> toString(678)
"678"

If you're using ES6 you could use template literals.
var a = 5;
console.log(`${a}`);

How do I get the decimal places of a floating point number in Javascript?

What I would like to have is the almost opposite of Number.prototype.toPrecision(), meaning that when i have number, how many decimals does it have? E.g.
(12.3456).getDecimals() // 4

For anyone wondering how to do this faster (without converting to string), here's a solution:
function precision(a) {
var e = 1;
while (Math.round(a * e) / e !== a) e *= 10;
return Math.log(e) / Math.LN10;
}
Edit: a more complete solution with edge cases covered:
function precision(a) {
if (!isFinite(a)) return 0;
var e = 1, p = 0;
while (Math.round(a * e) / e !== a) { e *= 10; p++; }
return p;
}

One possible solution (depends on the application):
var precision = (12.3456 + "").split(".")[1].length;

If by "precision" you mean "decimal places", then that's impossible because floats are binary. They don't have decimal places, and most values that have a small number of decimal places have recurring digits in binary, and when they're translated back to decimal that doesn't necessarily yield the original decimal number.
Any code that works with the "decimal places" of a float is liable to produce unexpected results on some numbers.

There is no native function to determine the number of decimals. What you can do is convert the number to string and then count the offset off the decimal delimiter .:
Number.prototype.getPrecision = function() {
var s = this + "",
d = s.indexOf('.') + 1;
return !d ? 0 : s.length - d;
};
(123).getPrecision() === 0;
(123.0).getPrecision() === 0;
(123.12345).getPrecision() === 5;
(1e3).getPrecision() === 0;
(1e-3).getPrecision() === 3;
But it's in the nature of floats to fool you. 1 may just as well be represented by 0.00000000989 or something. I'm not sure how well the above actually performs in real life applications.

Basing on #blackpla9ue comment and considering numbers exponential format:
function getPrecision (num) {
var numAsStr = num.toFixed(10); //number can be presented in exponential format, avoid it
numAsStr = numAsStr.replace(/0+$/g, '');
var precision = String(numAsStr).replace('.', '').length - num.toFixed().length;
return precision;
}
getPrecision(12.3456); //4
getPrecision(120.30003300000); //6, trailing zeros are truncated
getPrecision(15); //0
getPrecision(120.000)) //0
getPrecision(0.0000005); //7
getPrecision(-0.01)) //2

Try the following
function countDecimalPlaces(number) {
var str = "" + number;
var index = str.indexOf('.');
if (index >= 0) {
return str.length - index - 1;
} else {
return 0;
}
}

Based on #boolean_Type's method of handling exponents, but avoiding the regex:
function getPrecision (value) {
if (!isFinite(value)) { return 0; }
const [int, float = ''] = Number(value).toFixed(12).split('.');
let precision = float.length;
while (float[precision - 1] === '0' && precision >= 0) precision--;
return precision;
}

Here are a couple of examples, one that uses a library (BigNumber.js), and another that doesn't use a library. Assume you want to check that a given input number (inputNumber) has an amount of decimal places that is less than or equal to a maximum amount of decimal places (tokenDecimals).
With BigNumber.js
import BigNumber from 'bignumber.js'; // ES6
// const BigNumber = require('bignumber.js').default; // CommonJS
const tokenDecimals = 18;
const inputNumber = 0.000000000000000001;
// Convert to BigNumber
const inputNumberBn = new BigNumber(inputNumber);
// BigNumber.js API Docs: http://mikemcl.github.io/bignumber.js/#dp
console.log(`Invalid?: ${inputNumberBn.dp() > tokenDecimals}`);
Without BigNumber.js
function getPrecision(numberAsString) {
var n = numberAsString.toString().split('.');
return n.length > 1
? n[1].length
: 0;
}
const tokenDecimals = 18;
const inputNumber = 0.000000000000000001;
// Conversion of number to string returns scientific conversion
// So obtain the decimal places from the scientific notation value
const inputNumberDecimalPlaces = inputNumber.toString().split('-')[1];
// Use `toFixed` to convert the number to a string without it being
// in scientific notation and with the correct number decimal places
const inputNumberAsString = inputNumber.toFixed(inputNumberDecimalPlaces);
// Check if inputNumber is invalid due to having more decimal places
// than the permitted decimal places of the token
console.log(`Invalid?: ${getPrecision(inputNumberAsString) > tokenDecimals}`);

Assuming number is valid.
let number = 0.999;
let noOfPlaces = number.includes(".") //includes or contains
? number.toString().split(".").pop().length
: 0;

5622890.31 ops/s (91.58% slower):
function precision (n) {
return (n.toString().split('.')[1] || '').length
}
precision(1.0123456789)
33004904.53 ops/s (50.58% slower):
function precision (n) {
let e = 1
let p = 0
while(Math.round(n * e) / e !== n) {
e *= 10
p++
}
return p
}
precision(1.0123456789)
62610550.04 ops/s (6.25% slower):
function precision (n) {
let cur = n
let p = 0
while(!Number.isInteger(cur)) {
cur *= 10
p++
}
return p
}
precision(1.0123456789)
66786361.47 ops/s (fastest):
function precision (n) {
let cur = n
let p = 0
while(Math.floor(cur) !== cur) {
cur *= 10
p++
}
return p
}
precision(1.0123456789)

Here is a simple solution
First of all, if you pass a simple float value as 12.1234 then most of the below/above logics may work but if you pass a value as 12.12340, then it may exclude a count of 0. For e.g, if the value is 12.12340 then it may give you a result of 4 instead of 5. As per your problem statement, if you ask javascript to split and count your float value into 2 integers then it won't include trailing 0s of it.
Let's satisfy our requirement here with a trick ;)
In the below function you need to pass a value in string format and it will do your work
function getPrecision(value){
a = value.toString()
console.log('a ->',a)
b = a.split('.')
console.log('b->',b)
return b[1].length
getPrecision('12.12340') // Call a function
For an example, run the below logic
value = '12.12340'
a = value.toString()
b = a.split('.')
console.log('count of trailing decimals->',b[1].length)
That's it! It will give you the exact count for normal float values as well as the float values with trailing 0s!
Thank you!

This answer adds to Mourner's accepted solution by making the function more robust. As noted by many, floating point precision makes such a function unreliable. For example, precision(0.1+0.2) yields 17 rather than 1 (this might be computer specific, but for this example see https://jsfiddle.net/s0v17jby/5/).
IMHO, there are two ways around this: 1. either properly define a decimal type, using e.g. https://github.com/MikeMcl/decimal.js/, or 2. define an acceptable precision level which is both OK for your use case and not a problem for the js Number representation (8 bytes can safely represent a total of 16 digits AFAICT). For the latter workaround, one can write a more robust variant of the proposed function:
const MAX_DECIMAL_PRECISION = 9; /* must be <= 15 */
const maxDecimalPrecisionFloat = 10**MAX_DECIMAL_PRECISION;
function precisionRobust(a) {
if (!isFinite(a)) return 0;
var e = 1, p = 0;
while ( ++p<=MAX_DECIMAL_PRECISION && Math.round( ( Math.round(a * e) / e - a ) * maxDecimalPrecisionFloat ) !== 0) e *= 10;
return p-1;
}
In the above example, the maximum precision of 9 means this accepts up to 6 digits before the decimal point and 9 after (so this would work for numbers less than one million and with a maximum of 9 decimal points). If your use-case numbers are smaller then you can choose to make this precision even greater (but with a maximum of 15). It turns out that, for calculating precision, this function seems to do OK on larger numbers as well (though that would no longer be the case if we were, say, adding two rounded numbers within the precisionRobust function).
Finally, since we now know the maximum useable precision, we can further avoid infinite loops (which I have not been able to replicate but which still seem to cause problems for some).

Truncate (not round off) decimal numbers in javascript

I am trying to truncate decimal numbers to decimal places. Something like this:
5.467 -> 5.46
985.943 -> 985.94
toFixed(2) does just about the right thing but it rounds off the value. I don't need the value rounded off. Hope this is possible in javascript.

Dogbert's answer is good, but if your code might have to deal with negative numbers, Math.floor by itself may give unexpected results.
E.g. Math.floor(4.3) = 4, but Math.floor(-4.3) = -5
Use a helper function like this one instead to get consistent results:
truncateDecimals = function (number) {
return Math[number < 0 ? 'ceil' : 'floor'](number);
};
// Applied to Dogbert's answer:
var a = 5.467;
var truncated = truncateDecimals(a * 100) / 100; // = 5.46
Here's a more convenient version of this function:
truncateDecimals = function (number, digits) {
var multiplier = Math.pow(10, digits),
adjustedNum = number * multiplier,
truncatedNum = Math[adjustedNum < 0 ? 'ceil' : 'floor'](adjustedNum);
return truncatedNum / multiplier;
};
// Usage:
var a = 5.467;
var truncated = truncateDecimals(a, 2); // = 5.46
// Negative digits:
var b = 4235.24;
var truncated = truncateDecimals(b, -2); // = 4200
If that isn't desired behaviour, insert a call to Math.abs on the first line:
var multiplier = Math.pow(10, Math.abs(digits)),
EDIT: shendz correctly points out that using this solution with a = 17.56 will incorrectly produce 17.55. For more about why this happens, read What Every Computer Scientist Should Know About Floating-Point Arithmetic. Unfortunately, writing a solution that eliminates all sources of floating-point error is pretty tricky with javascript. In another language you'd use integers or maybe a Decimal type, but with javascript...
This solution should be 100% accurate, but it will also be slower:
function truncateDecimals (num, digits) {
var numS = num.toString(),
decPos = numS.indexOf('.'),
substrLength = decPos == -1 ? numS.length : 1 + decPos + digits,
trimmedResult = numS.substr(0, substrLength),
finalResult = isNaN(trimmedResult) ? 0 : trimmedResult;
return parseFloat(finalResult);
}
For those who need speed but also want to avoid floating-point errors, try something like BigDecimal.js. You can find other javascript BigDecimal libraries in this SO question: "Is there a good Javascript BigDecimal library?" and here's a good blog post about math libraries for Javascript

upd:
So, after all it turned out, rounding bugs will always haunt you, no matter how hard you try to compensate them. Hence the problem should be attacked by representing numbers exactly in decimal notation.
Number.prototype.toFixedDown = function(digits) {
var re = new RegExp("(\\d+\\.\\d{" + digits + "})(\\d)"),
m = this.toString().match(re);
return m ? parseFloat(m[1]) : this.valueOf();
};
[ 5.467.toFixedDown(2),
985.943.toFixedDown(2),
17.56.toFixedDown(2),
(0).toFixedDown(1),
1.11.toFixedDown(1) + 22];
// [5.46, 985.94, 17.56, 0, 23.1]
Old error-prone solution based on compilation of others':
Number.prototype.toFixedDown = function(digits) {
var n = this - Math.pow(10, -digits)/2;
n += n / Math.pow(2, 53); // added 1360765523: 17.56.toFixedDown(2) === "17.56"
return n.toFixed(digits);
}

var a = 5.467;
var truncated = Math.floor(a * 100) / 100; // = 5.46

You can fix the rounding by subtracting 0.5 for toFixed, e.g.
(f - 0.005).toFixed(2)

Nice one-line solution:
function truncate (num, places) {
return Math.trunc(num * Math.pow(10, places)) / Math.pow(10, places);
}
Then call it with:
truncate(3.5636232, 2); // returns 3.56
truncate(5.4332312, 3); // returns 5.433
truncate(25.463214, 4); // returns 25.4632

Consider taking advantage of the double tilde: ~~.
Take in the number. Multiply by significant digits after the decimal so that you can truncate to zero places with ~~. Divide that multiplier back out. Profit.
function truncator(numToTruncate, intDecimalPlaces) {
var numPower = Math.pow(10, intDecimalPlaces); // "numPowerConverter" might be better
return ~~(numToTruncate * numPower)/numPower;
}
I'm trying to resist wrapping the ~~ call in parens; order of operations should make that work correctly, I believe.
alert(truncator(5.1231231, 1)); // is 5.1
alert(truncator(-5.73, 1)); // is -5.7
alert(truncator(-5.73, 0)); // is -5
JSFiddle link.
EDIT: Looking back over, I've unintentionally also handled cases to round off left of the decimal as well.
alert(truncator(4343.123, -2)); // gives 4300.
The logic's a little wacky looking for that usage, and may benefit from a quick refactor. But it still works. Better lucky than good.

I thought I'd throw in an answer using | since it is simple and works well.
truncate = function(number, places) {
var shift = Math.pow(10, places);
return ((number * shift) | 0) / shift;
};

Truncate using bitwise operators:
~~0.5 === 0
~~(-0.5) === 0
~~14.32794823 === 14
~~(-439.93) === -439

#Dogbert's answer can be improved with Math.trunc, which truncates instead of rounding.
There is a difference between rounding and truncating. Truncating is
clearly the behaviour this question is seeking. If I call
truncate(-3.14) and receive -4 back, I would definitely call that
undesirable. – #NickKnowlson
var a = 5.467;
var truncated = Math.trunc(a * 100) / 100; // = 5.46
var a = -5.467;
var truncated = Math.trunc(a * 100) / 100; // = -5.46

I wrote an answer using a shorter method. Here is what I came up with
function truncate(value, precision) {
var step = Math.pow(10, precision || 0);
var temp = Math.trunc(step * value);
return temp / step;
}
The method can be used like so
truncate(132456.25456789, 5)); // Output: 132456.25456
truncate(132456.25456789, 3)); // Output: 132456.254
truncate(132456.25456789, 1)); // Output: 132456.2
truncate(132456.25456789)); // Output: 132456
Or, if you want a shorter syntax, here you go
function truncate(v, p) {
var s = Math.pow(10, p || 0);
return Math.trunc(s * v) / s;
}

I think this function could be a simple solution:
function trunc(decimal,n=2){
let x = decimal + ''; // string
return x.lastIndexOf('.')>=0?parseFloat(x.substr(0,x.lastIndexOf('.')+(n+1))):decimal; // You can use indexOf() instead of lastIndexOf()
}
console.log(trunc(-241.31234,2));
console.log(trunc(241.312,5));
console.log(trunc(-241.233));
console.log(trunc(241.2,0));
console.log(trunc(241));

Number.prototype.trim = function(decimals) {
var s = this.toString();
var d = s.split(".");
d[1] = d[1].substring(0, decimals);
return parseFloat(d.join("."));
}
console.log((5.676).trim(2)); //logs 5.67

I'm a bit confused as to why there are so many different answers to such a fundamentally simple question; there are only two approaches which I saw which seemed to be worth looking at. I did a quick benchmark to see the speed difference using https://jsbench.me/.
This is the solution which is currently (9/26/2020) flagged as the answer:
function truncate(n, digits) {
var re = new RegExp("(\\d+\\.\\d{" + digits + "})(\\d)"),
m = n.toString().match(re);
return m ? parseFloat(m[1]) : n.valueOf();
};
[ truncate(5.467,2),
truncate(985.943,2),
truncate(17.56,2),
truncate(0, 1),
truncate(1.11, 1) + 22];
However, this is doing string and regex stuff, which is usually not very efficient, and there is a Math.trunc function which does exactly what the OP wants just with no decimals. Therefore, you can easily use that plus a little extra arithmetic to get the same thing.
Here is another solution I found on this thread, which is the one I would use:
function truncate(n, digits) {
var step = Math.pow(10, digits || 0);
var temp = Math.trunc(step * n);
return temp / step;
}
[ truncate(5.467,2),
truncate(985.943,2),
truncate(17.56,2),
truncate(0, 1),
truncate(1.11, 1) + 22];
The first method is "99.92% slower" than the second, so the second is definitely the one I would recommend using.
Okay, back to finding other ways to avoid work...

I found a problem: considering the next situation: 2.1 or 1.2 or -6.4
What if you want always 3 decimals or two or wharever, so, you have to complete the leading zeros to the right
// 3 decimals numbers
0.5 => 0.500
// 6 decimals
0.1 => 0.10000
// 4 decimales
-2.1 => -2.1000
// truncate to 3 decimals
3.11568 => 3.115
This is the fixed function of Nick Knowlson
function truncateDecimals (num, digits)
{
var numS = num.toString();
var decPos = numS.indexOf('.');
var substrLength = decPos == -1 ? numS.length : 1 + decPos + digits;
var trimmedResult = numS.substr(0, substrLength);
var finalResult = isNaN(trimmedResult) ? 0 : trimmedResult;
// adds leading zeros to the right
if (decPos != -1){
var s = trimmedResult+"";
decPos = s.indexOf('.');
var decLength = s.length - decPos;
while (decLength <= digits){
s = s + "0";
decPos = s.indexOf('.');
decLength = s.length - decPos;
substrLength = decPos == -1 ? s.length : 1 + decPos + digits;
};
finalResult = s;
}
return finalResult;
};
https://jsfiddle.net/huttn155/7/

function toFixed(number, digits) {
var reg_ex = new RegExp("(\\d+\\.\\d{" + digits + "})(\\d)")
var array = number.toString().match(reg_ex);
return array ? parseFloat(array[1]) : number.valueOf()
}
var test = 10.123456789
var __fixed = toFixed(test, 6)
console.log(__fixed)
// => 10.123456

The answer by #kirilloid seems to be the correct answer, however, the main code needs to be updated. His solution doesn't take care of negative numbers (which someone did mention in the comment section but has not been updated in the main code).
Updating that to a complete final tested solution:
Number.prototype.toFixedDown = function(digits) {
var re = new RegExp("([-]*\\d+\\.\\d{" + digits + "})(\\d)"),
m = this.toString().match(re);
return m ? parseFloat(m[1]) : this.valueOf();
};
Sample Usage:
var x = 3.1415629;
Logger.log(x.toFixedDown(2)); //or use whatever you use to log
Fiddle: JS Number Round down
PS: Not enough repo to comment on that solution.

Here my take on the subject:
convert.truncate = function(value, decimals) {
decimals = (decimals === undefined ? 0 : decimals);
return parseFloat((value-(0.5/Math.pow(10, decimals))).toFixed(decimals),10);
};
It's just a slightly more elaborate version of
(f - 0.005).toFixed(2)

Here is simple but working function to truncate number upto 2 decimal places.
function truncateNumber(num) {
var num1 = "";
var num2 = "";
var num1 = num.split('.')[0];
num2 = num.split('.')[1];
var decimalNum = num2.substring(0, 2);
var strNum = num1 +"."+ decimalNum;
var finalNum = parseFloat(strNum);
return finalNum;
}

The resulting type remains a number...
/* Return the truncation of n wrt base */
var trunc = function(n, base) {
n = (n / base) | 0;
return base * n;
};
var t = trunc(5.467, 0.01);

Lodash has a few Math utility methods that can round, floor, and ceil a number to a given decimal precision. This leaves off trailing zeroes.
They take an interesting approach, using the exponent of a number. Apparently this avoids rounding issues.
(Note: func is Math.round or ceil or floor in the code below)
// Shift with exponential notation to avoid floating-point issues.
var pair = (toString(number) + 'e').split('e'),
value = func(pair[0] + 'e' + (+pair[1] + precision));
pair = (toString(value) + 'e').split('e');
return +(pair[0] + 'e' + (+pair[1] - precision));
Link to the source code

const TO_FIXED_MAX = 100;
function truncate(number, decimalsPrecison) {
// make it a string with precision 1e-100
number = number.toFixed(TO_FIXED_MAX);
// chop off uneccessary digits
const dotIndex = number.indexOf('.');
number = number.substring(0, dotIndex + decimalsPrecison + 1);
// back to a number data type (app specific)
return Number.parseFloat(number);
}
// example
truncate(0.00000001999, 8);
0.00000001
works with:
negative numbers
very small numbers (Number.EPSILON precision)

The one that is mark as the solution is the better solution I been found until today, but has a serious problem with 0 (for example, 0.toFixedDown(2) gives -0.01). So I suggest to use this:
Number.prototype.toFixedDown = function(digits) {
if(this == 0) {
return 0;
}
var n = this - Math.pow(10, -digits)/2;
n += n / Math.pow(2, 53); // added 1360765523: 17.56.toFixedDown(2) === "17.56"
return n.toFixed(digits);
}

Here is what I use:
var t = 1;
for (var i = 0; i < decimalPrecision; i++)
t = t * 10;
var f = parseFloat(value);
return (Math.floor(f * t)) / t;

You can work with strings.
It Checks if '.' exists, and then removes part of string.
truncate (7.88, 1) --> 7.8
truncate (7.889, 2) --> 7.89
truncate (-7.88, 1 ) --> -7.88
function truncate(number, decimals) {
const tmp = number + '';
if (tmp.indexOf('.') > -1) {
return +tmp.substr(0 , tmp.indexOf('.') + decimals+1 );
} else {
return +number
}
}

function trunc(num, dec) {
const pow = 10 ** dec
return Math.trunc(num * pow) / pow
}
// ex.
trunc(4.9634, 1) // 4.9
trunc(4.9634, 2) // 4.96
trunc(-4.9634, 1) // -4.9

You can use toFixed(2) to convert your float to a string with 2 decimal points. Then you can wrap that in floatParse() to convert that string back to a float to make it usable for calculations or db storage.
const truncatedNumber = floatParse(num.toFixed(2))
I am not sure of the potential drawbacks of this answer like increased processing time but I tested edge cases from other comments like .29 which returns .29 (not .28 like other solutions). It also handles negative numbers.

just to point out a simple solution that worked for me
convert it to string and then regex it...
var number = 123.45678;
var number_s = '' + number;
var number_truncated_s = number_s.match(/\d*\.\d{4}/)[0]
var number_truncated = parseFloat(number_truncated_s)
It can be abbreviated to
var number_truncated = parseFloat(('' + 123.4568908).match(/\d*\.\d{4}/)[0])

Here is an ES6 code which does what you want
const truncateTo = (unRouned, nrOfDecimals = 2) => {
const parts = String(unRouned).split(".");
if (parts.length !== 2) {
// without any decimal part
return unRouned;
}
const newDecimals = parts[1].slice(0, nrOfDecimals),
newString = `${parts[0]}.${newDecimals}`;
return Number(newString);
};
// your examples
console.log(truncateTo(5.467)); // ---> 5.46
console.log(truncateTo(985.943)); // ---> 985.94
// other examples
console.log(truncateTo(5)); // ---> 5
console.log(truncateTo(-5)); // ---> -5
console.log(truncateTo(-985.943)); // ---> -985.94

Suppose you want to truncate number x till n digits.
Math.trunc(x * pow(10,n))/pow(10,n);

Number.prototype.truncate = function(places) {
var shift = Math.pow(10, places);
return Math.trunc(this * shift) / shift;
};

Get decimal portion of a number with JavaScript

I have float numbers like 3.2 and 1.6.
I need to separate the number into the integer and decimal part. For example, a value of 3.2 would be split into two numbers, i.e. 3 and 0.2
Getting the integer portion is easy:
n = Math.floor(n);
But I am having trouble getting the decimal portion.
I have tried this:
remainder = n % 2; //obtem a parte decimal do rating
But it does not always work correctly.
The previous code has the following output:
n = 3.1 // gives remainder = 1.1
What I am missing here?

Use 1, not 2.
js> 2.3 % 1
0.2999999999999998

var decimal = n - Math.floor(n)
Although this won't work for minus numbers so we might have to do
n = Math.abs(n); // Change to positive
var decimal = n - Math.floor(n)

You could convert to string, right?
n = (n + "").split(".");

How is 0.2999999999999998 an acceptable answer? If I were the asker I would want an answer of .3. What we have here is false precision, and my experiments with floor, %, etc indicate that Javascript is fond of false precision for these operations. So I think the answers that are using conversion to string are on the right track.
I would do this:
var decPart = (n+"").split(".")[1];
Specifically, I was using 100233.1 and I wanted the answer ".1".

Here's how I do it, which I think is the most straightforward way to do it:
var x = 3.2;
int_part = Math.trunc(x); // returns 3
float_part = Number((x-int_part).toFixed(2)); // return 0.2

A simple way of doing it is:
var x = 3.2;
var decimals = x - Math.floor(x);
console.log(decimals); //Returns 0.20000000000000018
Unfortunately, that doesn't return the exact value. However, that is easily fixed:
var x = 3.2;
var decimals = x - Math.floor(x);
console.log(decimals.toFixed(1)); //Returns 0.2
You can use this if you don't know the number of decimal places:
var x = 3.2;
var decimals = x - Math.floor(x);
var decimalPlaces = x.toString().split('.')[1].length;
decimals = decimals.toFixed(decimalPlaces);
console.log(decimals); //Returns 0.2

Language independent way:
var a = 3.2;
var fract = a * 10 % 10 /10; //0.2
var integr = a - fract; //3
note that it correct only for numbers with one fractioanal lenght )

You can use parseInt() function to get the integer part than use that to extract the decimal part
var myNumber = 3.2;
var integerPart = parseInt(myNumber);
var decimalPart = myNumber - integerPart;
Or you could use regex like:
splitFloat = function(n){
const regex = /(\d*)[.,]{1}(\d*)/;
var m;
if ((m = regex.exec(n.toString())) !== null) {
return {
integer:parseInt(m[1]),
decimal:parseFloat(`0.${m[2]}`)
}
}
}

The following works regardless of the regional settings for decimal separator... on the condition only one character is used for a separator.
var n = 2015.15;
var integer = Math.floor(n).toString();
var strungNumber = n.toString();
if (integer.length === strungNumber.length)
return "0";
return strungNumber.substring(integer.length + 1);
It ain't pretty, but it's accurate.

If precision matters and you require consistent results, here are a few propositions that will return the decimal part of any number as a string, including the leading "0.". If you need it as a float, just add var f = parseFloat( result ) in the end.
If the decimal part equals zero, "0.0" will be returned. Null, NaN and undefined numbers are not tested.
1. String.split
var nstring = (n + ""),
narray = nstring.split("."),
result = "0." + ( narray.length > 1 ? narray[1] : "0" );
2. String.substring, String.indexOf
var nstring = (n + ""),
nindex = nstring.indexOf("."),
result = "0." + (nindex > -1 ? nstring.substring(nindex + 1) : "0");
3. Math.floor, Number.toFixed, String.indexOf
var nstring = (n + ""),
nindex = nstring.indexOf("."),
result = ( nindex > -1 ? (n - Math.floor(n)).toFixed(nstring.length - nindex - 1) : "0.0");
4. Math.floor, Number.toFixed, String.split
var nstring = (n + ""),
narray = nstring.split("."),
result = (narray.length > 1 ? (n - Math.floor(n)).toFixed(narray[1].length) : "0.0");
Here is a jsPerf link: https://jsperf.com/decpart-of-number/
We can see that proposition #2 is the fastest.

A good option is to transform the number into a string and then split it.
// Decimal number
let number = 3.2;
// Convert it into a string
let string = number.toString();
// Split the dot
let array = string.split('.');
// Get both numbers
// The '+' sign transforms the string into a number again
let firstNumber = +array[0]; // 3
let secondNumber = +array[1]; // 2
In one line of code
let [firstNumber, secondNumber] = [+number.toString().split('.')[0], +number.toString().split('.')[1]];

Depending the usage you will give afterwards, but this simple solution could also help you.
Im not saying its a good solution, but for some concrete cases works
var a = 10.2
var c = a.toString().split(".")
console.log(c[1] == 2) //True
console.log(c[1] === 2) //False
But it will take longer than the proposed solution by #Brian M. Hunt
(2.3 % 1).toFixed(4)

I am using:
var n = -556.123444444;
var str = n.toString();
var decimalOnly = 0;
if( str.indexOf('.') != -1 ){ //check if has decimal
var decimalOnly = parseFloat(Math.abs(n).toString().split('.')[1]);
}
Input: -556.123444444
Result: 123444444

You could convert it to a string and use the replace method to replace the integer part with zero, then convert the result back to a number :
var number = 123.123812,
decimals = +number.toString().replace(/^[^\.]+/,'0');

n = Math.floor(x);
remainder = x % 1;

Math functions are faster, but always returns not native expected values.
Easiest way that i found is
(3.2+'').replace(/^[-\d]+\./, '')

The best way to avoid mathematical imprecision is to convert to a string, but ensure that it is in the "dot" format you expect by using toLocaleString:
function getDecimals(n) {
// Note that maximumSignificantDigits defaults to 3 so your decimals will be rounded if not changed.
const parts = n.toLocaleString('en-US', { maximumSignificantDigits: 18 }).split('.')
return parts.length > 1 ? Number('0.' + parts[1]) : 0
}
console.log(getDecimals(10.58))

You can simply use parseInt() function to help, example:
let decimal = 3.2;
let remainder = decimal - parseInt(decimal);
document.write(remainder);

I had a case where I knew all the numbers in question would have only one decimal and wanted to get the decimal portion as an integer so I ended up using this kind of approach:
var number = 3.1,
decimalAsInt = Math.round((number - parseInt(number)) * 10); // returns 1
This works nicely also with integers, returning 0 in those cases.

Although I am very late to answer this, please have a look at the code.
let floatValue = 3.267848;
let decimalDigits = floatValue.toString().split('.')[1];
let decimalPlaces = decimalDigits.length;
let decimalDivider = Math.pow(10, decimalPlaces);
let fractionValue = decimalDigits/decimalDivider;
let integerValue = floatValue - fractionValue;
console.log("Float value: "+floatValue);
console.log("Integer value: "+integerValue);
console.log("Fraction value: "+fractionValue)

I like this answer https://stackoverflow.com/a/4512317/1818723 just need to apply float point fix
function fpFix(n) {
return Math.round(n * 100000000) / 100000000;
}
let decimalPart = 2.3 % 1; //0.2999999999999998
let correct = fpFix(decimalPart); //0.3
Complete function handling negative and positive
function getDecimalPart(decNum) {
return Math.round((decNum % 1) * 100000000) / 100000000;
}
console.log(getDecimalPart(2.3)); // 0.3
console.log(getDecimalPart(-2.3)); // -0.3
console.log(getDecimalPart(2.17247436)); // 0.17247436
P.S. If you are cryptocurrency trading platform developer or banking system developer or any JS developer ;) please apply fpFix everywhere. Thanks!

2021 Update
Optimized version that tackles precision (or not).
// Global variables.
const DEFAULT_PRECISION = 16;
const MAX_CACHED_PRECISION = 20;
// Helper function to avoid numerical imprecision from Math.pow(10, x).
const _pow10 = p => parseFloat(`1e+${p}`);
// Cache precision coefficients, up to a precision of 20 decimal digits.
const PRECISION_COEFS = new Array(MAX_CACHED_PRECISION);
for (let i = 0; i !== MAX_CACHED_PRECISION; ++i) {
PRECISION_COEFS[i] = _pow10(i);
}
// Function to get a power of 10 coefficient,
// optimized for both speed and precision.
const pow10 = p => PRECISION_COEFS[p] || _pow10(p);
// Function to trunc a positive number, optimized for speed.
// See: https://stackoverflow.com/questions/38702724/math-floor-vs-math-trunc-javascript
const trunc = v => (v < 1e8 && ~~v) || Math.trunc(v);
// Helper function to get the decimal part when the number is positive,
// optimized for speed.
// Note: caching 1 / c or 1e-precision still leads to numerical errors.
// So we have to pay the price of the division by c.
const _getDecimals = (v = 0, precision = DEFAULT_PRECISION) => {
const c = pow10(precision); // Get precision coef.
const i = trunc(v); // Get integer.
const d = v - i; // Get decimal.
return Math.round(d * c) / c;
}
// Augmenting Number proto.
Number.prototype.getDecimals = function(precision) {
return (isFinite(this) && (precision ? (
(this < 0 && -_getDecimals(-this, precision))
|| _getDecimals(this, precision)
) : this % 1)) || 0;
}
// Independent function.
const getDecimals = (input, precision) => (isFinite(input) && (
precision ? (
(this < 0 && -_getDecimals(-this, precision))
|| _getDecimals(this, precision)
) : this % 1
)) || 0;
// Tests:
const test = (value, precision) => (
console.log(value, '|', precision, '-->', value.getDecimals(precision))
);
test(1.001 % 1); // --> 0.0009999999999998899
test(1.001 % 1, 16); // --> 0.000999999999999
test(1.001 % 1, 15); // --> 0.001
test(1.001 % 1, 3); // --> 0.001
test(1.001 % 1, 2); // --> 0
test(-1.001 % 1, 16); // --> -0.000999999999999
test(-1.001 % 1, 15); // --> -0.001
test(-1.001 % 1, 3); // --> -0.001
test(-1.001 % 1, 2); // --> 0

After looking at several of these, I am now using...
var rtnValue = Number(7.23);
var tempDec = ((rtnValue / 1) - Math.floor(rtnValue)).toFixed(2);

Floating-point decimal sign and number format can be dependent from country (.,), so independent solution, which preserved floating point part, is:
getFloatDecimalPortion = function(x) {
x = Math.abs(parseFloat(x));
let n = parseInt(x);
return Number((x - n).toFixed(Math.abs((""+x).length - (""+n).length - 1)));
}
– it is internationalized solution, instead of location-dependent:
getFloatDecimalPortion = x => parseFloat("0." + ((x + "").split(".")[1]));
Solution desription step by step:
parseFloat() for guaranteeing input cocrrection
Math.abs() for avoiding problems with negative numbers
n = parseInt(x) for getting decimal part
x - n for substracting decimal part
We have now number with zero decimal part, but JavaScript could give us additional floating part digits, which we do not want
So, limit additional digits by calling toFixed() with count of digits in floating part of original float number x. Count is calculated as difference between length of original number x and number n in their string representation.

This function splits float number into integers and returns it in array:
function splitNumber(num)
{
num = (""+num).match(/^(-?[0-9]+)([,.][0-9]+)?/)||[];
return [ ~~num[1], +(0+num[2])||0 ];
}
console.log(splitNumber(3.02)); // [ 3, 0.2 ]
console.log(splitNumber(123.456)); // [ 123, 0.456 ]
console.log(splitNumber(789)); // [ 789, 0 ]
console.log(splitNumber(-2.7)); // [ -2, 0.7 ]
console.log(splitNumber("test")); // [ 0, 0 ]
You can extend it to only return existing numbers and null if no number exists:
function splitNumber(num)
{
num = (""+num).match(/^(-?[0-9]+)([,.][0-9]+)?/);
return [ num ? ~~num[1] : null, num && num[2] ? +(0 + num[2]) : null ];
}
console.log(splitNumber(3.02)); // [ 3, 0.02 ]
console.log(splitNumber(123.456)); // [ 123, 0.456 ]
console.log(splitNumber(789)); // [ 789, null ]
console.log(splitNumber(-2.7)); // [ -2, 0.7 ]
console.log(splitNumber("test")); // [ null, null ]

You can also truncate the number
function decimals(val) {
const valStr = val.toString();
const valTruncLength = String(Math.trunc(val)).length;
const dec =
valStr.length != valTruncLength
? valStr.substring(valTruncLength + 1)
: "";
return dec;
}
console.log("decimals: ", decimals(123.654321));
console.log("no decimals: ", decimals(123));

The following function will return an array which will have 2 elements. The first element will be the integer part and the second element will be the decimal part.
function splitNum(num) {
num = num.toString().split('.')
num[0] = Number(num[0])
if (num[1]) num[1] = Number('0.' + num[1])
else num[1] = 0
return num
}
//call this function like this
let num = splitNum(3.2)
console.log(`Integer part is ${num[0]}`)
console.log(`Decimal part is ${num[1]}`)
//or you can call it like this
let [int, deci] = splitNum(3.2)
console.log('Intiger part is ' + int)
console.log('Decimal part is ' + deci)

For example for add two numbers
function add(number1, number2) {
let decimal1 = String(number1).substring(String(number1).indexOf(".") + 1).length;
let decimal2 = String(number2).substring(String(number2).indexOf(".") + 1).length;
let z = Math.max(decimal1, decimal2);
return (number1 * Math.pow(10, z) + number2 * Math.pow(10, z)) / Math.pow(10, z);
}

float a=3.2;
int b=(int)a; // you'll get output b=3 here;
int c=(int)a-b; // you'll get c=.2 value here

How can I pad a value with leading zeros?

What is the recommended way to zerofill a value in JavaScript? I imagine I could build a custom function to pad zeros on to a typecasted value, but I'm wondering if there is a more direct way to do this?
Note: By "zerofilled" I mean it in the database sense of the word (where a 6-digit zerofilled representation of the number 5 would be "000005").

I can't believe all the complex answers on here... Just use this:
var zerofilled = ('0000'+n).slice(-4);
let n = 1
var zerofilled = ('0000'+n).slice(-4);
console.log(zerofilled)

Simple way. You could add string multiplication for the pad and turn it into a function.
var pad = "000000";
var n = '5';
var result = (pad+n).slice(-pad.length);
As a function,
function paddy(num, padlen, padchar) {
var pad_char = typeof padchar !== 'undefined' ? padchar : '0';
var pad = new Array(1 + padlen).join(pad_char);
return (pad + num).slice(-pad.length);
}
var fu = paddy(14, 5); // 00014
var bar = paddy(2, 4, '#'); // ###2

Since ECMAScript 2017 we have padStart:
const padded = (.1 + "").padStart(6, "0");
console.log(`-${padded}`);
Before ECMAScript 2017
With toLocaleString:
var n=-0.1;
var res = n.toLocaleString('en', {minimumIntegerDigits:4,minimumFractionDigits:2,useGrouping:false});
console.log(res);

I actually had to come up with something like this recently.
I figured there had to be a way to do it without using loops.
This is what I came up with.
function zeroPad(num, numZeros) {
var n = Math.abs(num);
var zeros = Math.max(0, numZeros - Math.floor(n).toString().length );
var zeroString = Math.pow(10,zeros).toString().substr(1);
if( num < 0 ) {
zeroString = '-' + zeroString;
}
return zeroString+n;
}
Then just use it providing a number to zero pad:
> zeroPad(50,4);
"0050"
If the number is larger than the padding, the number will expand beyond the padding:
> zeroPad(51234, 3);
"51234"
Decimals are fine too!
> zeroPad(51.1234, 4);
"0051.1234"
If you don't mind polluting the global namespace you can add it to Number directly:
Number.prototype.leftZeroPad = function(numZeros) {
var n = Math.abs(this);
var zeros = Math.max(0, numZeros - Math.floor(n).toString().length );
var zeroString = Math.pow(10,zeros).toString().substr(1);
if( this < 0 ) {
zeroString = '-' + zeroString;
}
return zeroString+n;
}
And if you'd rather have decimals take up space in the padding:
Number.prototype.leftZeroPad = function(numZeros) {
var n = Math.abs(this);
var zeros = Math.max(0, numZeros - n.toString().length );
var zeroString = Math.pow(10,zeros).toString().substr(1);
if( this < 0 ) {
zeroString = '-' + zeroString;
}
return zeroString+n;
}
Cheers!
XDR came up with a logarithmic variation that seems to perform better.
WARNING: This function fails if num equals zero (e.g. zeropad(0, 2))
function zeroPad (num, numZeros) {
var an = Math.abs (num);
var digitCount = 1 + Math.floor (Math.log (an) / Math.LN10);
if (digitCount >= numZeros) {
return num;
}
var zeroString = Math.pow (10, numZeros - digitCount).toString ().substr (1);
return num < 0 ? '-' + zeroString + an : zeroString + an;
}
Speaking of performance, tomsmeding compared the top 3 answers (4 with the log variation). Guess which one majorly outperformed the other two? :)

Modern browsers now support padStart, you can simply now do:
string.padStart(maxLength, "0");
Example:
string = "14";
maxLength = 5; // maxLength is the max string length, not max # of fills
res = string.padStart(maxLength, "0");
console.log(res); // prints "00014"
number = 14;
maxLength = 5; // maxLength is the max string length, not max # of fills
res = number.toString().padStart(maxLength, "0");
console.log(res); // prints "00014"

Here's what I used to pad a number up to 7 characters.
("0000000" + number).slice(-7)
This approach will probably suffice for most people.
Edit: If you want to make it more generic you can do this:
("0".repeat(padding) + number).slice(-padding)
Edit 2: Note that since ES2017 you can use String.prototype.padStart:
number.toString().padStart(padding, "0")

Unfortunately, there are a lot of needless complicated suggestions for this problem, typically involving writing your own function to do math or string manipulation or calling a third-party utility. However, there is a standard way of doing this in the base JavaScript library with just one line of code. It might be worth wrapping this one line of code in a function to avoid having to specify parameters that you never want to change like the local name or style.
var amount = 5;
var text = amount.toLocaleString('en-US',
{
style: 'decimal',
minimumIntegerDigits: 3,
useGrouping: false
});
This will produce the value of "005" for text. You can also use the toLocaleString function of Number to pad zeros to the right side of the decimal point.
var amount = 5;
var text = amount.toLocaleString('en-US',
{
style: 'decimal',
minimumFractionDigits: 2,
useGrouping: false
});
This will produce the value of "5.00" for text. Change useGrouping to true to use comma separators for thousands.
Note that using toLocaleString() with locales and options arguments is standardized separately in ECMA-402, not in ECMAScript. As of today, some browsers only implement basic support, i.e. toLocaleString() may ignore any arguments.
Complete Example

If the fill number is known in advance not to exceed a certain value, there's another way to do this with no loops:
var fillZeroes = "00000000000000000000"; // max number of zero fill ever asked for in global
function zeroFill(number, width) {
// make sure it's a string
var input = number + "";
var prefix = "";
if (input.charAt(0) === '-') {
prefix = "-";
input = input.slice(1);
--width;
}
var fillAmt = Math.max(width - input.length, 0);
return prefix + fillZeroes.slice(0, fillAmt) + input;
}
Test cases here: http://jsfiddle.net/jfriend00/N87mZ/

The quick and dirty way:
y = (new Array(count + 1 - x.toString().length)).join('0') + x;
For x = 5 and count = 6 you'll have y = "000005"

Here's a quick function I came up with to do the job. If anyone has a simpler approach, feel free to share!
function zerofill(number, length) {
// Setup
var result = number.toString();
var pad = length - result.length;
while(pad > 0) {
result = '0' + result;
pad--;
}
return result;
}

ECMAScript 2017:
use padStart or padEnd
'abc'.padStart(10); // " abc"
'abc'.padStart(10, "foo"); // "foofoofabc"
'abc'.padStart(6,"123465"); // "123abc"
More info:
https://github.com/tc39/proposal-string-pad-start-end
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/padStart

I often use this construct for doing ad-hoc padding of some value n, known to be a positive, decimal:
(offset + n + '').substr(1);
Where offset is 10^^digits.
E.g., padding to 5 digits, where n = 123:
(1e5 + 123 + '').substr(1); // => 00123
The hexadecimal version of this is slightly more verbose:
(0x100000 + 0x123).toString(16).substr(1); // => 00123
Note 1: I like #profitehlolz's solution as well, which is the string version of this, using slice()'s nifty negative-index feature.

I really don't know why, but no one did it in the most obvious way. Here it's my implementation.
Function:
/** Pad a number with 0 on the left */
function zeroPad(number, digits) {
var num = number+"";
while(num.length < digits){
num='0'+num;
}
return num;
}
Prototype:
Number.prototype.zeroPad=function(digits){
var num=this+"";
while(num.length < digits){
num='0'+num;
}
return(num);
};
Very straightforward, I can't see any way how this can be any simpler. For some reason I've seem many times here on SO, people just try to avoid 'for' and 'while' loops at any cost. Using regex will probably cost way more cycles for such a trivial 8 digit padding.

In all modern browsers you can use
numberStr.padStart(numberLength, "0");
function zeroFill(num, numLength) {
var numberStr = num.toString();
return numberStr.padStart(numLength, "0");
}
var numbers = [0, 1, 12, 123, 1234, 12345];
numbers.forEach(
function(num) {
var numString = num.toString();
var paddedNum = zeroFill(numString, 5);
console.log(paddedNum);
}
);
Here is the MDN reference https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/padStart

I use this snippet to get a five-digits representation:
(value+100000).toString().slice(-5) // "00123" with value=123

The power of Math!
x = integer to pad
y = number of zeroes to pad
function zeroPad(x, y)
{
y = Math.max(y-1,0);
var n = (x / Math.pow(10,y)).toFixed(y);
return n.replace('.','');
}

This is the ES6 solution.
function pad(num, len) {
return '0'.repeat(len - num.toString().length) + num;
}
alert(pad(1234,6));

Not that this question needs more answers, but I thought I would add the simple lodash version of this.
_.padLeft(number, 6, '0')

I didn't see anyone point out the fact that when you use String.prototype.substr() with a negative number it counts from the right.
A one liner solution to the OP's question, a 6-digit zerofilled representation of the number 5, is:
console.log(("00000000" + 5).substr(-6));
Generalizing we'll get:
function pad(num, len) { return ("00000000" + num).substr(-len) };
console.log(pad(5, 6));
console.log(pad(45, 6));
console.log(pad(345, 6));
console.log(pad(2345, 6));
console.log(pad(12345, 6));

Don't reinvent the wheel; use underscore string:
jsFiddle
var numToPad = '5';
alert(_.str.pad(numToPad, 6, '0')); // Yields: '000005'

After a, long, long time of testing 15 different functions/methods found in this questions answers, I now know which is the best (the most versatile and quickest).
I took 15 functions/methods from the answers to this question and made a script to measure the time taken to execute 100 pads. Each pad would pad the number 9 with 2000 zeros. This may seem excessive, and it is, but it gives you a good idea about the scaling of the functions.
The code I used can be found here:
https://gist.github.com/NextToNothing/6325915
Feel free to modify and test the code yourself.
In order to get the most versatile method, you have to use a loop. This is because with very large numbers others are likely to fail, whereas, this will succeed.
So, which loop to use? Well, that would be a while loop. A for loop is still fast, but a while loop is just slightly quicker(a couple of ms) - and cleaner.
Answers like those by Wilco, Aleksandar Toplek or Vitim.us will do the job perfectly.
Personally, I tried a different approach. I tried to use a recursive function to pad the string/number. It worked out better than methods joining an array but, still, didn't work as quick as a for loop.
My function is:
function pad(str, max, padder) {
padder = typeof padder === "undefined" ? "0" : padder;
return str.toString().length < max ? pad(padder.toString() + str, max, padder) : str;
}
You can use my function with, or without, setting the padding variable. So like this:
pad(1, 3); // Returns '001'
// - Or -
pad(1, 3, "x"); // Returns 'xx1'
Personally, after my tests, I would use a method with a while loop, like Aleksandar Toplek or Vitim.us. However, I would modify it slightly so that you are able to set the padding string.
So, I would use this code:
function padLeft(str, len, pad) {
pad = typeof pad === "undefined" ? "0" : pad + "";
str = str + "";
while(str.length < len) {
str = pad + str;
}
return str;
}
// Usage
padLeft(1, 3); // Returns '001'
// - Or -
padLeft(1, 3, "x"); // Returns 'xx1'
You could also use it as a prototype function, by using this code:
Number.prototype.padLeft = function(len, pad) {
pad = typeof pad === "undefined" ? "0" : pad + "";
var str = this + "";
while(str.length < len) {
str = pad + str;
}
return str;
}
// Usage
var num = 1;
num.padLeft(3); // Returns '001'
// - Or -
num.padLeft(3, "x"); // Returns 'xx1'

First parameter is any real number, second parameter is a positive integer specifying the minimum number of digits to the left of the decimal point and third parameter is an optional positive integer specifying the number if digits to the right of the decimal point.
function zPad(n, l, r){
return(a=String(n).match(/(^-?)(\d*)\.?(\d*)/))?a[1]+(Array(l).join(0)+a[2]).slice(-Math.max(l,a[2].length))+('undefined'!==typeof r?(0<r?'.':'')+(a[3]+Array(r+1).join(0)).slice(0,r):a[3]?'.'+a[3]:''):0
}
so
zPad(6, 2) === '06'
zPad(-6, 2) === '-06'
zPad(600.2, 2) === '600.2'
zPad(-600, 2) === '-600'
zPad(6.2, 3) === '006.2'
zPad(-6.2, 3) === '-006.2'
zPad(6.2, 3, 0) === '006'
zPad(6, 2, 3) === '06.000'
zPad(600.2, 2, 3) === '600.200'
zPad(-600.1499, 2, 3) === '-600.149'

The latest way to do this is much simpler:
var number = 2
number.toLocaleString(undefined, {minimumIntegerDigits:2})
output: "02"

Just another solution, but I think it's more legible.
function zeroFill(text, size)
{
while (text.length < size){
text = "0" + text;
}
return text;
}

This one is less native, but may be the fastest...
zeroPad = function (num, count) {
var pad = (num + '').length - count;
while(--pad > -1) {
num = '0' + num;
}
return num;
};

My solution
Number.prototype.PadLeft = function (length, digit) {
var str = '' + this;
while (str.length < length) {
str = (digit || '0') + str;
}
return str;
};
Usage
var a = 567.25;
a.PadLeft(10); // 0000567.25
var b = 567.25;
b.PadLeft(20, '2'); // 22222222222222567.25

With ES6+ JavaScript:
You can "zerofill a number" with something like the following function:
/**
* #param number The number
* #param minLength Minimal length for your string with leading zeroes
* #return Your formatted string
*/
function zerofill(nb, minLength) {
// Convert your number to string.
let nb2Str = nb.toString()
// Guess the number of zeroes you will have to write.
let nbZeroes = Math.max(0, minLength - nb2Str.length)
// Compute your result.
return `${ '0'.repeat(nbZeroes) }${ nb2Str }`
}
console.log(zerofill(5, 6)) // Displays "000005"
With ES2017+:
/**
* #param number The number
* #param minLength Minimal length for your string with leading zeroes
* #return Your formatted string
*/
const zerofill = (nb, minLength) => nb.toString().padStart(minLength, '0')
console.log(zerofill(5, 6)) // Displays "000005"

Use recursion:
function padZero(s, n) {
s = s.toString(); // In case someone passes a number
return s.length >= n ? s : padZero('0' + s, n);
}

Some monkeypatching also works
String.prototype.padLeft = function (n, c) {
if (isNaN(n))
return null;
c = c || "0";
return (new Array(n).join(c).substring(0, this.length-n)) + this;
};
var paddedValue = "123".padLeft(6); // returns "000123"
var otherPadded = "TEXT".padLeft(8, " "); // returns " TEXT"

function pad(toPad, padChar, length){
return (String(toPad).length < length)
? new Array(length - String(toPad).length + 1).join(padChar) + String(toPad)
: toPad;
}
pad(5, 0, 6) = 000005
pad('10', 0, 2) = 10 // don't pad if not necessary
pad('S', 'O', 2) = SO
...etc.
Cheers