PHP's sprintf in JavaScript

How to use

You you can install via yarn add locutus and require this function via const sprintf = require('locutus/php/strings/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("%01.2f", 123.1)	'123.10'
2	sprintf("[%10s]", 'monkey')	'[ monkey]'
3	sprintf("[%'#10s]", 'monkey')	'[####monkey]'
4	sprintf("%d", 123456789012345)	'123456789012345'
5	sprintf('%-03s', 'E')	'E00'
6	sprintf('%+010d', 9)	'+000000009'
7	sprintf('%+0\'@10d', 9)	'@@@@@@@@+9'
8	sprintf('%.f', 3.14)	'3.140000'
9	sprintf('%% %2$d', 1, 2)	'% 2'

Here’s what our current JavaScript equivalent to PHP's sprintf looks like.

module.exports = function sprintf() {
  //  discuss at: https://locutus.io/php/sprintf/
  // original by: Ash Searle (https://hexmen.com/blog/)
  // improved by: Michael White (https://getsprink.com)
  // improved by: Jack
  // improved by: Kevin van Zonneveld (https://kvz.io)
  // improved by: Kevin van Zonneveld (https://kvz.io)
  // improved by: Kevin van Zonneveld (https://kvz.io)
  // improved by: Dj
  // improved by: Allidylls
  //    input by: Paulo Freitas
  //    input by: Brett Zamir (https://brett-zamir.me)
  // improved by: Rafał Kukawski (https://kukawski.pl)
  //   example 1: sprintf("%01.2f", 123.1)
  //   returns 1: '123.10'
  //   example 2: sprintf("[%10s]", 'monkey')
  //   returns 2: '[    monkey]'
  //   example 3: sprintf("[%'#10s]", 'monkey')
  //   returns 3: '[####monkey]'
  //   example 4: sprintf("%d", 123456789012345)
  //   returns 4: '123456789012345'
  //   example 5: sprintf('%-03s', 'E')
  //   returns 5: 'E00'
  //   example 6: sprintf('%+010d', 9)
  //   returns 6: '+000000009'
  //   example 7: sprintf('%+0\'@10d', 9)
  //   returns 7: '@@@@@@@@+9'
  //   example 8: sprintf('%.f', 3.14)
  //   returns 8: '3.140000'
  //   example 9: sprintf('%% %2$d', 1, 2)
  //   returns 9: '% 2'

  const regex = /%%|%(?:(\d+)\$)?((?:[-+#0 ]|'[\s\S])*)(\d+)?(?:\.(\d*))?([\s\S])/g
  const args = arguments
  let i = 0
  const format = args[i++]

  const _pad = function (str, len, chr, leftJustify) {
    if (!chr) {
      chr = ' '
    }
    const padding = str.length >= len ? '' : new Array((1 + len - str.length) >>> 0).join(chr)
    return leftJustify ? str + padding : padding + str
  }

  const justify = function (value, prefix, leftJustify, minWidth, padChar) {
    const diff = minWidth - value.length
    if (diff > 0) {
      // when padding with zeros
      // on the left side
      // keep sign (+ or -) in front
      if (!leftJustify && padChar === '0') {
        value = [value.slice(0, prefix.length), _pad('', diff, '0', true), value.slice(prefix.length)].join('')
      } else {
        value = _pad(value, minWidth, padChar, leftJustify)
      }
    }
    return value
  }

  const _formatBaseX = function (value, base, leftJustify, minWidth, precision, padChar) {
    // Note: casts negative numbers to positive ones
    const number = value >>> 0
    value = _pad(number.toString(base), precision || 0, '0', false)
    return justify(value, '', leftJustify, minWidth, padChar)
  }

  // _formatString()
  const _formatString = function (value, leftJustify, minWidth, precision, customPadChar) {
    if (precision !== null && precision !== undefined) {
      value = value.slice(0, precision)
    }
    return justify(value, '', leftJustify, minWidth, customPadChar)
  }

  // doFormat()
  const doFormat = function (substring, argIndex, modifiers, minWidth, precision, specifier) {
    let number, prefix, method, textTransform, value

    if (substring === '%%') {
      return '%'
    }

    // parse modifiers
    let padChar = ' ' // pad with spaces by default
    let leftJustify = false
    let positiveNumberPrefix = ''
    let j, l

    for (j = 0, l = modifiers.length; j < l; j++) {
      switch (modifiers.charAt(j)) {
        case ' ':
        case '0':
          padChar = modifiers.charAt(j)
          break
        case '+':
          positiveNumberPrefix = '+'
          break
        case '-':
          leftJustify = true
          break
        case "'":
          if (j + 1 < l) {
            padChar = modifiers.charAt(j + 1)
            j++
          }
          break
      }
    }

    if (!minWidth) {
      minWidth = 0
    } else {
      minWidth = +minWidth
    }

    if (!isFinite(minWidth)) {
      throw new Error('Width must be finite')
    }

    if (!precision) {
      precision = specifier === 'd' ? 0 : 'fFeE'.indexOf(specifier) > -1 ? 6 : undefined
    } else {
      precision = +precision
    }

    if (argIndex && +argIndex === 0) {
      throw new Error('Argument number must be greater than zero')
    }

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

    value = argIndex ? args[+argIndex] : args[i++]

    switch (specifier) {
      case '%':
        return '%'
      case 's':
        return _formatString(value + '', leftJustify, minWidth, precision, padChar)
      case 'c':
        return _formatString(String.fromCharCode(+value), leftJustify, minWidth, precision, padChar)
      case 'b':
        return _formatBaseX(value, 2, leftJustify, minWidth, precision, padChar)
      case 'o':
        return _formatBaseX(value, 8, leftJustify, minWidth, precision, padChar)
      case 'x':
        return _formatBaseX(value, 16, leftJustify, minWidth, precision, padChar)
      case 'X':
        return _formatBaseX(value, 16, leftJustify, minWidth, precision, padChar).toUpperCase()
      case 'u':
        return _formatBaseX(value, 10, leftJustify, minWidth, precision, padChar)
      case 'i':
      case 'd':
        number = +value || 0
        // Plain Math.round doesn't just truncate
        number = Math.round(number - (number % 1))
        prefix = number < 0 ? '-' : positiveNumberPrefix
        value = prefix + _pad(String(Math.abs(number)), precision, '0', false)

        if (leftJustify && padChar === '0') {
          // can't right-pad 0s on integers
          padChar = ' '
        }
        return justify(value, prefix, leftJustify, minWidth, padChar)
      case 'e':
      case 'E':
      case 'f': // @todo: Should handle locales (as per setlocale)
      case 'F':
      case 'g':
      case 'G':
        number = +value
        prefix = number < 0 ? '-' : positiveNumberPrefix
        method = ['toExponential', 'toFixed', 'toPrecision']['efg'.indexOf(specifier.toLowerCase())]
        textTransform = ['toString', 'toUpperCase']['eEfFgG'.indexOf(specifier) % 2]
        value = prefix + Math.abs(number)[method](precision)
        return justify(value, prefix, leftJustify, minWidth, padChar)[textTransform]()
      default:
        // unknown specifier, consume that char and return empty
        return ''
    }
  }

  try {
    return format.replace(regex, doFormat)
  } catch (err) {
    return false
  }
}

A community effort

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.

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