ECMAScript proposal: {BigInt,Number}.fromString


This proposal is at stage 1 of the TC39 process.


The BigInt proposal initially included a static BigInt.parseInt method. After some discussion, it was removed in favor of a separate proposal to add a static fromString method to both BigInt and Number. This is that proposal.


{BigInt,Number}.prototype.toString(radix) enables converting a numeric value into a string representation of that value. For BigInts specifically, there is currently no built-in way to do the inverse, i.e. to turn a string representation of a BigInt with a given radix back into a BigInt.

For Number values, there is parseInt(string, radix = 10) and Number.parseInt(string, radix = 10), but its behavior is suboptimal:

  • It returns NaN instead of throwing a SyntaxError exception when string does not represent a number.
  • It returns NaN instead of throwing a RangeError exception when radix is not valid (i.e. radix !== 0 && radix < 2 or radix > 36).
  • It accepts radix 0, treating it as 10 instead, which does not make sense.
  • It ignores leading whitespace and trailing non-digit characters.
  • It supports hexadecimal integer literal prefixes 0x and 0X but lacks support for octal integer literal prefixes 0o and 0O or binary integer literal prefixes 0b and 0B, which is inconsistent.
  • The fact that parseInt has some level of support for integer literal prefixes means that it’s not a clear counterpart to toString.

Proposed solution

We propose extending both BigInt and Number with a new static fromString(string, radix = 10) method which acts as the inverse of {BigInt,Number}.prototype.toString(radix = 10). It accepts only strings that can be produced by {BigInt,Number}.prototype.toString(radix = 10), and throws an exception for any other input.

High-level API

// → 42
Number.fromString('42', 10);
// → 42

// → 42n
BigInt.fromString('42', 10);
// → 42n

Illustrative examples

The following examples use Number.fromString. The semantics for BigInt.fromString are identical except it returns a BigInt rather than a Number.

Unlike parseInt, fromString intentionally lacks special handling for integer literal prefixes.

// → 12648430
// → 0
// → 0

// → SyntaxError
// → SyntaxError
// → SyntaxError

Number.fromString('C0FFEE', 16);
// → SyntaxError (toString produces lowercase digits)
Number.fromString('c0ffee', 16);
// → 12648430 === 0xc0ffee
Number.fromString('755', 8);
// → 493 === 0o755
Number.fromString('00101010', 2);
// → 42 === 0b00101010

Unlike parseInt, fromString throws a SyntaxError exception when string does not represent a number.

// → NaN
Number.parseInt(' \n ');
// → NaN
// → NaN

// → SyntaxError
Number.fromString(' \n ');
// → SyntaxError
// → SyntaxError

Unlike parseInt, fromString throws a RangeError exception when radix < 2 or radix > 36.

Number.parseInt('1234', 0);
// → 1234
Number.parseInt('1234', 1);
// → NaN
Number.parseInt('1234', 37);
// → NaN

Number.fromString('1234', 0);
// → RangeError
Number.fromString('1234', 1);
// → RangeError
Number.fromString('1234', 37);
// → RangeError

Unlike parseInt, fromString throws a TypeError exception when string is not a string.

Number.parseInt(true, 32);
// → 978894

Number.fromString(true, 32);
// → TypeError


What about legacy octal integers?

fromString intentionally lacks special handling for legacy octal integer literals, i.e. those without the explicit 0o or 0O prefix such as 010. In other words, Number.fromString('010') throws a SyntaxError exception.

What about numeric separators?

fromString does not need to support numeric separators, as they cannot occur in {BigInt,Number}.prototype.toString(radix) output. Number.fromString('1_000_000_000') throws a SyntaxError exception.

Does BigInt.fromString(string) support the n suffix?

BigInt.fromString does not need to support the n suffix used for BigInt literals, as it doesn’t occur in BigInt.prototype.toString(radix) output. Furthermore, supporting it would introduce an ambiguity for radices where n is a valid digit: should BigInt.fromString('1n', 32) return 1 or 55? With the current proposal, BigInt.fromString('1n', 32) returns 55, and BigInt.fromString('1n') throws a SyntaxError exception.



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