C's stdlib.strtod in TypeScript

How to use

Install via yarn add locutus and import: import { strtod } from 'locutus/c/stdlib/strtod'.

Or with CommonJS: const { strtod } = require('locutus/c/stdlib/strtod')

Use a bundler that supports tree-shaking so you only ship the functions you actually use. Vite, webpack, Rollup, and Parcel all handle this. For server-side use this is less of a concern.

Examples

These examples are extracted from test cases that automatically verify our functions against their native counterparts.

#	code	expected result
1	strtod('3.14')	3.14
2	strtod(' -2.5e2xyz')	-250
3	strtod('abc')	0

C types and TypeScript/JavaScript

C is statically typed while TypeScript/JavaScript is dynamically typed. Locutus C functions accept TypeScript/JavaScript's flexible types but are only parity-verified for inputs that would be valid in C.

For example, abs() in TypeScript/JavaScript accepts floats (like C's fabs()) and handles strings gracefully, but only integer inputs are verified against native C. This pragmatic approach gives you the expected C behavior for valid inputs while leveraging TypeScript/JavaScript's flexibility for edge cases.

Notes

• Parses a floating-point number from the start of the string.

• Stops at the first invalid trailing character, similar to C strtod.

Here's what our current TypeScript equivalent to C's strtod found in the stdlib.h header file looks like.

TSTypeScript JSJavaScript TSStandalone TS JSStandalone JS

Copy code

export function strtod(str: string): number {
  //      discuss at: https://locutus.io/c/stdlib/strtod/
  // parity verified: C 23
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Parses a floating-point number from the start of the string.
  //          note 2: Stops at the first invalid trailing character, similar to C strtod.
  //       example 1: strtod('3.14')
  //       returns 1: 3.14
  //       example 2: strtod('  -2.5e2xyz')
  //       returns 2: -250
  //       example 3: strtod('abc')
  //       returns 3: 0

  const input = String(str).replace(/^\s+/, '')
  const match = input.match(/^[+-]?(?:\d+\.?\d*|\.\d+)(?:[eE][+-]?\d+)?/)
  if (!match || !match[0]) {
    return 0
  }

  const parsed = Number.parseFloat(match[0])
  return Number.isNaN(parsed) ? 0 : parsed
}

export function strtod(str) {
  //      discuss at: https://locutus.io/c/stdlib/strtod/
  // parity verified: C 23
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Parses a floating-point number from the start of the string.
  //          note 2: Stops at the first invalid trailing character, similar to C strtod.
  //       example 1: strtod('3.14')
  //       returns 1: 3.14
  //       example 2: strtod('  -2.5e2xyz')
  //       returns 2: -250
  //       example 3: strtod('abc')
  //       returns 3: 0

  const input = String(str).replace(/^\s+/, '')
  const match = input.match(/^[+-]?(?:\d+\.?\d*|\.\d+)(?:[eE][+-]?\d+)?/)
  if (!match || !match[0]) {
    return 0
  }

  const parsed = Number.parseFloat(match[0])
  return Number.isNaN(parsed) ? 0 : parsed
}

Improve this function

Locutus is a community effort following The McDonald's Theory: we ship first iterations, hoping others will improve them. If you see something that could be better, we'd love your contribution.

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