Develop Reference

Passing a number to parseFloat() instead of string

In my code, the value of a particular var can originate from any one of a number of different json sources. For some of those sources, the json element concerned will be a string (e.g. "temp": "10.2"), while for other sources the json element will already be a float (e.g. "temp": 10.2).
Does it do any harm (is anything likely to break) if I just pass the json element (from whatever source) through a parseFloat(), even if it's already a float? It seems to work; I'm just thinking about good/bad practice and possible breakage in future or on a different platform.
Thanks.

You should be able to call parseFloat() on a float or a string without any problems. If it is a float already, it's converted to a string first, and then to a float again, so it's a little less efficient, but it shouldn't matter too much.
You should still check the result for NaN, in case there's something unexpected in the data.

The most appropriate method to convert any datatype to a number is to use the Number function:
In a non-constructor context (i.e., without the new operator),
Number can be used to perform a type conversion.
Number("1234") // 1234
Number(1234) // 1234
This method differs from parseFloat in these ways at least:
Number function does not perform "double-conversion" if the input is already a number (ref)
Parse float converts the input to a string then extracts the number (ref)
Number function returns common sense values for most datatypes e.g. Number(true) yields 1
Parse float uses the string value of input so parseFloat(true) tries to parse number from "true" and yields NaN
Number function fails when input string is an invalid number e.g. Number("123abc") yields NaN
Parse float tries to parse as much of a number as possible e.g. parseFloat("123abc") yields 123

If you are sure the value is always a valid number, you should use Number(stringOrNumber).
If you need some additional safety using parseFloat() you could also write your own function which is also performance optimized:
function toFloat(value) {
return typeof value === 'number' ? value : parseFloat(value);
}
I also created a jsPerf test case that shows the performance is >30% better than the plain parseFloat() for a 1:1 ratio between strings and numbers as input values.

Nope there is no problem with passing a number to it
MDN says as long as it can be converted to a number, nothing breaking should happen.
If the first character cannot be converted to a number, parseFloat returns NaN.
As an alternative, you could use the unary operator + which does basically the same thing as parseFloat and also returns NaN if it didn't work.
For instance:
var myFloat = +('10.5');
var myOtherFloat = parseFloat('10.5', 10);
var alreadyAFloat = parseFloat(10.5, 10);
console.log(myFloat === myOtherFloat && myOtherFloat === alreadyAFloat); // true

Wether it's a float or a String using parseFloat() is much safer to avoid all kind of errors.
As you said it will always work, but if you enforce it to be a float you will avoid getting any Exception.
For Example:
Both parseFloat('10.2', 10) and parseFloat(10.2, 10) will work
perfectly and will give you the same result which is 10.2.

Personally I can't see this being a problem what so ever, to be honest I would always use the parsefloat() for one reason, and that is safety. You can never be to sure what may happen, so always predict the worse :D

Getting the numeric value after the hyphen in a string

How can I extract and get just the numeric value after the hyphen in a string?
Here is the input string:
var x = "-2147467259"
After some processing.... return:
alert(2147467259)
How do I accomplish this?

You could replace away the hyphen:
alert(+x.replace("-", ""));
And yes, the + is important. It converts a string to a number; so you're removing the hypen by replacing it with nothing, and then essentially casting the result of that operation into a number. This operation will also work if no hyphen is present.
You could also use substr to achieve this:
alert(+x.substr(1));
You could also use parseInt to convert the string to a number (which will end up negative if a hyphen is persent), and then find its absolute value:
alert(Math.abs(parseInt(x, 10));
As Bergi notes, if you can be sure that the first character in the string is always a hyphen, you can simple return its negative, which will by default cast the value into a number and then perform the negative operation on it:
alert(-x);
You could also check to see if the number is negative or positive via a tertiary operator and then perform the respective operation on it to ensure that it is a positive Number:
x = x >= 0 ? +x : -x;
This may be cheaper in terms of performance than using Math.abs, but the difference will be minuscule either way.
As you can see, there really are a variety of ways to achieve this. I'd recommend reading up on JavaScript string functions and number manipulation in general, as well as examining JavaScript's Math object to get a feel for what tools are available to you when you go to solve a problem.

How about:
Math.abs(parseInt("-2147467259"))
Or
"-2147467259".replace('-','')
or
"-2147467259".replace(/\-/,'')
#1 option is converting the string to numbers. The #2 approach is removing all - from the string and the #3 option even though it will not be necessary on this example uses Regular Expression but I wanted to show the possibility of using RegEx in replace situations.
If you need a number as the final value #1 is your choice if you need strings #2 is your choice.

Should literal numbers not have quotes?

I know that literal numbers do not require quotes around the value. For instance, var x=123; is acceptable and does not need to be var x='123'; or var x="123";.
That being said, is there anything wrong with quoting a literal number?
If the "number" was a zipcode or database record ID, and not a number in the normal sense which might be used in arithmetic, would the answer be different?

It isn't a number. Quoting a number makes it a string, which can make for some differences in the way they're handled. For example:
var a = 1;
var b = '33';
console.log(a + b === 34); // false
console.log(a + b === '34'); // true
Strings also have different types and methods for manipulating them. However, for most of the numeric operators (-, /, *, and other bitwise operators), they convert the string form to its numeric equivalent before performing the operation.
There are also a few differences where numbers are not stored with their exact value in some cases, due to the nature of the floating point format JavaScript numbers are stored in. Strings avoid this problem, though it is much harder to manipulate them. Converting these back to numbers reintroduces these issues. For example, see this:
var recordID = 9007199254740992;
var previousID = recordID;
recordID += 1;
console.log(recordID === previousID); // true

Adding quotes makes the number a string literal and so serves a different purpose than the Number literal defined without quotes.
JavaScript has the concept of type coercion which might have confused you.

Quoting makes a string of a number. It means that for example + operation will concatenate instead of add:
var a = 'asdf';
var b = '20';
var c = a + b; // asdf20
Here is a great explanation of what is going on.

I know that literal numbers do not require quotes around the value. For instance, var x=123; is acceptable and does not need to be var x='123'; or var x="123";.
It's not a matter of required Vs not required (optional)
Using quotes (single or double) you state that it is a string (a sequence of characters - no matter if they're all digits)
If you don't place quotes you state it is a number.
That being said, is there anything wrong with quoting a literal number?
No if the entity it represents is not actually a number but a string. So...
If the "number" was a zipcode or database record ID, and not a number in the normal sense which might be used in arithmetic, would the answer be different?
If the number is a zipcode it may make sense to put quotes, because it is a "code", not a number and is not subject to arithmetics operations.
You're not going to divide a zipcode by 2 or sum two zipcodes because that would not make sense.
But instead of deciding to use quotes or not based on what the value represents I suggest you to consider the problem from the language perspective
You should understand and keep always in mind how do the language's operators behave when you use a string instead of a number in an expression (assignment or comparison).

Javascript convert string to integer

I am just dipping my toe into the confusing world of javascript, more out of necessity than desire and I have come across a problem of adding two integers.
1,700.00 + 500.00
returns 1,700.00500.00
So after some research I see that 1,700.00 is being treated as a string and that I need to convert it.
The most relevant pages I read to resolve this were this question and this page. However when I use
parseInt(string, radix)
it returns 1. Am I using the wrong function or the an incorrect radix (being honest I can't get my head around how I decide which radix to use).
var a="1,700.00";
var b=500.00;
parseInt(a, 10);

Basic Answer
The reason parseInt is not working is because of the comma. You could remove the comma using a regex such as:
var num = '1,700.00';
num = num.replace(/\,/g,'');
This will return a string with a number in it. Now you can parseInt. If you do not choose a radix it will default to 10 which was the correct value to use here.
num = parseInt(num);
Do this for each of your string numbers before adding them and everything should work.
More information
How the replace works:
More information on replace at mdn:
`/` - start
`\,` - escaped comma
`/` - end
`g` - search globally
The global search will look for all matches (it would stop after the first match without this)
'' replace the matched sections with an empty string, essentially deleting them.
Regular Expressions
A great tool to test regular expressions: Rubular and more info about them at mdn
If you are looking for a good tutorial here is one.
ParseInt and Rounding, parseFloat
parseInt always rounds to the nearest integer. If you need decimal places there are a couple of tricks you can use. Here is my favorite:
2 places: `num = parseInt(num * 100) / 100;`
3 places: `num = parseInt(num * 1000) / 1000;`
For more information on parseInt look at mdn.
parseFloat could also be used if you do not want rounding. I assumed you did as the title was convert to an integer. A good example of this was written by #fr0zenFry below. He pointed out that parseFloat also does not take a radix so it is always in base10. For more info see mdn.

Try using replace() to replace a , with nothing and then parseFloat() to get the number as float. From the variables in OP, it appears that there may be fractional numbers too, so, parseInt() may not work well in such cases(digits after decimal will be stripped off).
Use regex inside replace() to get rid of each appearance of ,.
var a = parseFloat('1,700.00'.replace(/,/g, ''));
var b = parseFloat('500.00'.replace(/,/g, ''));
var sum = a+b;
This should give you correct result even if your number is fractional like 1,700.55.
If I go by the title of your question, you need an integer. For this you can use parseInt(string, radix). It works without a radix but it is always a good idea to specify this because you never know how browsers may behave(for example, see comment #Royi Namir). This function will round off the string to nearest integer value.
var a = parseInt('1,700.00'.replace(/,/g, ''), 10); //radix 10 will return base10 value
var b = parseInt('500.00'.replace(/,/g, ''), 10);
var sum = a+b;
Note that a radix is not required in parseFloat(), it will always return a decimal/base10 value. Also, it will it will strip off any extra zeroes at the end after decimal point(ex: 17500.50 becomes 17500.5 and 17500.00 becomes 17500). If you need to get 2 decimal places always, append another function toFixed(decimal places).
var a = parseFloat('1,700.00'.replace(/,/g, ''));
var b = parseFloat('500.00'.replace(/,/g, ''));
var sum = (a+b).toFixed(2); //change argument in toFixed() as you need
// 2200.00
Another alternative to this was given by #EpiphanyMachine which will need you to multiply and then later divide every value by 100. This may become a problem if you want to change decimal places in future, you will have to change multiplication/division factor for every variable. With toFixed(), you just change the argument. But remember that toFixed() changes the number back to string unlike #EpiphanyMachine solution. So you will be your own judge.

try this :
parseFloat(a.replace(/,/g, ''));
it will work also on : 1,800,300.33
Example :
parseFloat('1,700,800.010'.replace(/,/g, '')) //1700800.01

Javascript doesn't understand that comma. Remove it like this:
a.replace(',', '')
Once you've gotten rid of the comma, the string should be parsed with no problem.

why is 10 * (7/10) = 7 in JavaScript?

7 and 10 in the expression (7/10) are integers, so the result 0.7 should be integer as well, which is 0, and the result for the entire expression should be 0 too. However, it's giving me the result of 7, why? Is it ignoring the parentheses or converts to double automatically?

JavaScript doesn't distinguish between integers and floating point numbers, everything I believe is considered a double so that is just why you get the result.
Take a look at the details on the Number property on MDN.

JavaScript doesn't have an integer type, or a double, or a float... it just has 1 type for all numbers: the helpfuly called Number type (try var foo = new Number(7);, or var foo = Number('123string');
Now, I know I said that JS doesn't know of floats, but that's not entirely true. All Number type vars/values are, essentially 64 bit floats, as defined by the IEEE 754 standard (which are, indeed, as Jan Dvorak kindly pointed out to me, double's in most staticly typed languages), with all the caveats that brings with it:
(.1 + .2);//0.30000000000000004
But that's not the point. The point is that, in JS you can perform float + int arithmatic without there ever being a need for internal casts, or conversions. That's why 10*(7/10) will always be 7

There is no int and double in JavaScript
In JavaScript, both int, flot, and double are normalized to work together. They are treated as 1 (They're treated as as Number, which is an IEEE 754 float. Thanks #Elias Van Ootegem). Equality, Liberty and Fraternity. and thus;
10*0.7 = 7
JavaScript is not like C.

Javascript doesn't have integers, and even if it did, there's nothing that says that / needs to return an integer (just because another language may do that doesn't mean every language has to). The operation results in a float/Number, just like all Javascript numbers are, period.

try this
10*parseInt(7/10)
hope this will help you

If you try to follow the rules, then
10 * (7/10) --> 10 * .7 --> 7
You cannot change the way its gonna result into.
so the result 0.7 should be integer as well, which is 0
If you want this, then try using
Math.Floor();
This would change the decimals to the nearest int! Or try out parse()

JavaScript uses dynamic types. That means that a variable like this:
var str = "hi";
Can later become:
str = 123; //now we have an 'int'
str += 0.35; //now str is 123.35, a 'float'
So JavaScript doesn't cast floats to ints for example.
If you want to force a "cast" then you have to do:
var integer = parseInt( 3.14*9.0291+23, 10 ); //the second parameter (10) is the 'base'
But remember, Javascript will not take care of types, that's your problem.