number to string conversion error in javascript

Question

Did you ever try to convert a big number to a string in javascript?

Please try this:

var n = 10152557636804775;
console.log(n); // outputs 10152557636804776

Can you help me understand why?

Answer 1

10152557636804775 is higher than the maximum integer number that can be safely represented in JavaScript (it's Number.MAX_SAFE_INTEGER). See also this post for more details.

From MDN (emphasis is mine):

The MAX_SAFE_INTEGER constant has a value of 9007199254740991. The reasoning behind that number is that JavaScript uses double-precision floating-point format numbers as specified in IEEE 754 and can only safely represent numbers between -(2^53 - 1) and 2^53 - 1.

To check if a given variable can be safely represented as an integer (without representation errors) you can use IsSafeInteger():

var n = 10152557636804775;
console.assert(Number.isSafeInteger(n) == false);