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Using Javascript parseInt returns a different value

I'm not entirely sure why the string "6145390195186705543" is outputting 6145390195186705000, at first reading through the threads it may be the base radix but even tinkering around with that still gives me the same results, can anyone help explain because I do believe this is not a bug, but I'm not entirely sure what's the explanation here.
const digits = [6,1,4,5,3,9,0,1,9,5,1,8,6,7,0,5,5,4,3]
const val1 = digits.join('') // string "6145390195186705543"
const test1 = Number(val1) // outputs 6145390195186705000
const test2 = parseInt(val1) // outputs 6145390195186705000

It's not an issue with parseInt. If you were to create the same number from a number literal, you'd get the same problem.
This happens because JavaScript stores numbers as double prescision floats, which offer approximately 16 significant decimal digits.
The only way to fix this is to not use the Number type. JavaScript has another number type, BigInt, which offers arbitrary precision integers. Here's how you can do it with it:
const digits = [6,1,4,5,3,9,0,1,9,5,1,8,6,7,0,5,5,4,3]
const val1 = digits.join('')
const test3 = BigInt(val1) // 6145390195186705543n - The n at the end means it's a BigInt

Determine precision of number, including trailing zeros

I suspect that this isn't possible, but giving it a try:
Is it possible to determine the number of decimal points of a number, whether or not those decimal points are trailing zeros?
getPrecision(3) // 0
getPrecision(3.2) // 1
getPrecision(2.30) // 2
I've found a couple of solutions for the first two cases (including this: How do I get the decimal places of a floating point number in Javascript?) but nothing for the last case. Converting 3.20 to a string results in "3.2", which doesn't help, and I'm about out of ideas.

Yes, this is possible but only with a trick
First of all, 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! Just pass your float value in string format and you are done!
Thank you!

How to convert BigInt to Number in JavaScript?

I found myself in the situation where I wanted to convert a BigInt value to a Number value. Knowing that my value is a safe integer, how can I convert it?

Turns out it's as easy as passing it to the Number constructor:
const myBigInt = BigInt(10); // `10n` also works
const myNumber = Number(myBigInt);
Of course, you should bear in mind that your BigInt value must be within [Number.MIN_SAFE_INTEGER, Number.MAX_SAFE_INTEGER] for the conversion to work properly, as stated in the question.

You can use parseInt or Number
const large = BigInt(309);
const b = parseInt(large);
console.log(b);
const n = Number(large);
console.log(n);

Edit: see the discussion in the comments below as to why this answer is not correct. I am leaving the answer up regardless for disambiguation.
You should use either of the static methods:
BigInt.asIntN() - Clamps a BigInt value to a signed integer value, and returns that value.
BigInt.asUintN() - Clamps a BigInt value to an unsigned integer value, and returns that value.
as documented here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt#static_methods

Why I can't convert string to number without losing precision in JS?

We all know that +, Number() and parseInt() can convert string to integer.
But in my case I have very weird result.
I need to convert string '6145390195186705543' to number.
let str = '6145390195186705543';
let number = +str; // 6145390195186705000, but should be: 6145390195186705543
Could someone explain why and how to solve it?

Your number is above the Number.MAX_SAFE_INTEGER (9,007,199,254,740,991), meaning js might have a problem to represent it well.
More information

You are outside the maximum range. Check in your console by typing Number.MAX_SAFE_INTEGER
If you want a number outside this range, take a look into BigInt that allows to define numbers beyond the safe range
https://developers.google.com/web/updates/2018/05/bigint
Read the documentation well before using it since the usage is different than usual

I am guessing this is to solve the plusOne problem in leetcode. As others have answered, you cannot store value higher than the max safe integer. However you can write logic to add values manually.
If you want to add one to the number represented in the array, you can use the below function. If you need to add a different value, you need to tweak the solution a bit.
var plusOne = function(digits) {
let n = digits.length, carry=0;
if(digits[n-1]<9){
digits[n-1] +=1;
} else{
digits[n-1] = 0;
carry=1;
for(let i=n-2;i>=0;i--){
if(digits[i]<9){
digits[i]+=1;
carry=0;
break;
}else{
digits[i]=0;
}
}
if(carry>0){
digits.unshift(carry);
}
}
return digits;
};

Short answer: Your string represents a number to large to fit into the JavaScript number container.
According to the javascript documentation the maximum safe number is 2^53 which is 9007199254740992 source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number
When you try and convert your number you're creating an overflow exception so you get weird results.

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.