C's stdio.sprintf in JavaScript

How to use

You you can install via yarn add locutus and require this function via const sprintf = require('locutus/c/stdio/sprintf').

It is important to use a bundler that supports tree-shaking so that you only ship the functions that you actually use to your browser, instead of all of Locutus, which is massive. Examples are: Parcel, webpack, or rollup.js. For server-side use this is typically less of a concern.

Examples

Please note that these examples are distilled from test cases that automatically verify our functions still work correctly. This could explain some quirky ones.

#	code	expected result
1	sprintf('%+10.*d', 5, 1)	' +00001'
2	sprintf('%s is a %d%% %s %s.', 'Param', 90, 'good', 'boy')	'Param is a 90% good boy.'

C types and JavaScript

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

For example, abs() in 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 JavaScript's flexibility for edge cases.

Here's what our current JavaScript equivalent to C's sprintf found in the stdio.h header file looks like.

function pad(str, minLength, padChar, leftJustify) {
  const diff = minLength - str.length
  const padStr = padChar.repeat(Math.max(0, diff))

  return leftJustify ? str + padStr : padStr + str
}

module.exports = function sprintf(format, ...args) {
  // original by: Rafał Kukawski
  // bugfixed by: Param Siddharth
  //   example 1: sprintf('%+10.*d', 5, 1)
  //   returns 1: '    +00001'
  //   example 2: sprintf('%s is a %d%% %s %s.', 'Param', 90, 'good', 'boy')
  //   returns 2: 'Param is a 90% good boy.'
  const placeholderRegex = /%(?:(\d+)\$)?([-+#0 ]*)(\*|\d+)?(?:\.(\*|\d*))?([\s\S])/g

  let index = 0

  return format.replace(placeholderRegex, function (match, param, flags, width, prec, modifier) {
    const leftJustify = flags.includes('-')

    // flag '0' is ignored when flag '-' is present
    const padChar = leftJustify ? ' ' : flags.split('').reduce((pc, c) => ([' ', '0'].includes(c) ? c : pc), ' ')

    const positiveSign = flags.includes('+') ? '+' : flags.includes(' ') ? ' ' : ''

    const minWidth = width === '*' ? args[index++] : +width || 0
    let precision = prec === '*' ? args[index++] : +prec

    if (param && !+param) {
      throw new Error('Param index must be greater than 0')
    }

    if (param && +param > args.length) {
      throw new Error('Too few arguments')
    }

    // compiling with default clang params, mixed positional and non-positional params
    // give only a warning
    const arg = param ? args[param - 1] : args[index]

    if (modifier !== '%') {
      index++
    }

    if (precision === undefined || isNaN(precision)) {
      precision = 'eEfFgG'.includes(modifier) ? 6 : modifier === 's' ? String(arg).length : undefined
    }

    switch (modifier) {
      case '%':
        return '%'
      case 'd':
      case 'i': {
        const number = Math.trunc(+arg || 0)
        const abs = Math.abs(number)
        const prefix = number < 0 ? '-' : positiveSign

        const str = pad(abs.toString(), precision || 0, '0', false)

        if (padChar === '0') {
          return prefix + pad(str, minWidth - prefix.length, padChar, leftJustify)
        }

        return pad(prefix + str, minWidth, padChar, leftJustify)
      }
      case 'e':
      case 'E':
      case 'f':
      case 'F':
      case 'g':
      case 'G': {
        const number = +arg
        const abs = Math.abs(number)
        const prefix = number < 0 ? '-' : positiveSign

        const op = [Number.prototype.toExponential, Number.prototype.toFixed, Number.prototype.toPrecision][
          'efg'.indexOf(modifier.toLowerCase())
        ]

        const tr = [String.prototype.toLowerCase, String.prototype.toUpperCase]['eEfFgG'.indexOf(modifier) % 2]

        const isSpecial = isNaN(abs) || !isFinite(abs)

        const str = isSpecial ? abs.toString().substr(0, 3) : op.call(abs, precision)

        if (padChar === '0' && !isSpecial) {
          return prefix + pad(tr.call(str), minWidth - prefix.length, padChar, leftJustify)
        }

        return pad(tr.call(prefix + str), minWidth, isSpecial ? ' ' : padChar, leftJustify)
      }
      case 'b':
      case 'o':
      case 'u':
      case 'x':
      case 'X': {
        const number = +arg || 0
        const intVal = Math.trunc(number) + (number < 0 ? 0xffffffff + 1 : 0)
        const base = [2, 8, 10, 16, 16]['bouxX'.indexOf(modifier)]
        const prefix = intVal && flags.includes('#') ? ['', '0', '', '0x', '0X']['bouxXX'.indexOf(modifier)] : ''

        if (padChar === '0' && prefix) {
          return (
            prefix +
            pad(pad(intVal.toString(base), precision, '0', false), minWidth - prefix.length, padChar, leftJustify)
          )
        }

        return pad(prefix + pad(intVal.toString(base), precision, '0', false), minWidth, padChar, leftJustify)
      }
      case 'p':
      case 'n': {
        throw new Error(`'${modifier}' modifier not supported`)
      }
      case 's': {
        return pad(String(arg).substr(0, precision), minWidth, padChar, leftJustify)
      }
      case 'c': {
        // extension, if arg is string, take first char
        const chr = typeof arg === 'string' ? arg.charAt(0) : String.fromCharCode(+arg)
        return pad(chr, minWidth, padChar, leftJustify)
      }
      case 'a':
      case 'A':
        throw new Error(`'${modifier}' modifier not yet implemented`)
      default:
        // for unknown modifiers, return the modifier char
        return modifier
    }
  })
}

Think you can do better?

Not unlike Wikipedia, Locutus is an ongoing community effort. Our philosophy follows The McDonald’s Theory. This means that we assimilate first iterations with imperfections, hoping for others to take issue with-and improve them. This unorthodox approach has worked very well to foster fun and fruitful collaboration, but please be reminded to use our creations at your own risk. THE SOFTWARE IS PROVIDED "AS IS" has never been more true than for Locutus.

Now go and: [ View on GitHub | Edit on GitHub | View Raw ]

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Ehm.. only 18 C functions?

We can use your help porting more. Have a rainy Sunday afternoon to spare and a taste for a porting puzzle?

• Get inspiration from the C math.h documentation, the C math.h source.
• Click "New file" in the appropriate folder on GitHub. This will fork the project to your account, directly add the file to it, and send a Pull Request to us.

We will then review it. If it's useful to the project and in line with our contributing guidelines your work will become part of Locutus and you'll be automatically credited in the authors section accordingly.

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