Subtracting long numbers in javascript

Question

Why is q == 0 in the following script?

<script>
  var start = 1234567890123456789;
  var end =   1234567890123456799;
  var q = end - start;
  alert(q);
</script>

I would think the result should be 10. What is the correct way to subtract these two numbers?

Answer 1

As of January 2020, BigInt datatype is going to be added to Javascript. The proposal is currently in Stage 4. It will enable precise calculation for number which are more than 2^53-1 (Number.MAX_SAFE_INTEGER).

BigInt has been shipped in Chrome, Node, Firefox, and is underway in Safari. Read more here.

var start = BigInt('1234567890123456789');
var end = BigInt('1234567890123456799');
var q = end - start;
alert(q)

A BigInt is created by appending n to the end of an integer literal — 10n — or by calling the function BigInt(). It is also different from Number so 1 + 1n will fail.

You can read more about it here from MDN pages

Answer 2

Because numbers in JavaScript are floating-point. They have limited precision.

When JavaScript sees a very long number, it rounds it to the nearest number it can represent as a 64-bit float. In your script, start and end get rounded to the same value.

alert(1234567890123456789);   // says: 1234567890123456800
alert(1234567890123456799);   // says: 1234567890123456800

There's no built-in way to do precise arithmetic on large integers, but you can use a BigInteger library such as this one.