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https://github.com/tc39/proposal-number-fromstring

{BigInt,Number}.fromString(string, radix)
https://github.com/tc39/proposal-number-fromstring

ecmascript

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{BigInt,Number}.fromString(string, radix)

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# ECMAScript proposal: `{BigInt,Number}.fromString`



## Status



This proposal is at stage 1 of [the TC39 process](https://tc39.github.io/process-document/).



## Background



[The `BigInt` proposal](https://github.com/tc39/proposal-bigint) initially included a static `BigInt.parseInt` method. After [some discussion](https://github.com/tc39/proposal-bigint/issues/86), it was [removed](https://github.com/tc39/proposal-bigint/commit/30b5594c30bdf7973323d61504017933673df283) in favor of a separate proposal to add a static `fromString` method to both `BigInt` and `Number`. This is that proposal.



## Motivation



`{BigInt,Number}.prototype.toString(radix)` enables converting a numeric value into a string representation of that value. For `BigInt`s 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)`](https://tc39.github.io/ecma262/#sec-parseint-string-radix) and [`Number.parseInt(string, radix = 10)`](https://tc39.github.io/ecma262/#sec-number.parseint), 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



```js

Number.fromString('42');

// → 42

Number.fromString('42', 10);

// → 42



BigInt.fromString('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.



```js

Number.parseInt('0xc0ffee');

// → 12648430

Number.parseInt('0o755');

// → 0

Number.parseInt('0b00101010');

// → 0



Number.fromString('0xc0ffee');

// → SyntaxError

Number.fromString('0o755');

// → SyntaxError

Number.fromString('0b00101010');

// → 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.



```js

Number.parseInt('');

// → NaN

Number.parseInt(' \n ');

// → NaN

Number.parseInt('x');

// → NaN



Number.fromString('');

// → SyntaxError

Number.fromString(' \n ');

// → SyntaxError

Number.fromString('x');

// → SyntaxError

```



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



```js

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.



```js

Number.parseInt(true, 32);

// → 978894



Number.fromString(true, 32);

// → TypeError

```



### FAQ



#### 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](https://github.com/tc39/proposal-numeric-separator), 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.



## Specification



* [Ecmarkup source](https://github.com/mathiasbynens/proposal-number-fromstring/blob/master/spec.html)

* [HTML version](https://mathiasbynens.github.io/proposal-number-fromstring/)



## Implementations



* none yet