Swift's String.padding in TypeScript

How to use

Install via yarn add locutus and import: import { padding } from 'locutus/swift/String/padding'.

Or with CommonJS: const { padding } = require('locutus/swift/String/padding')

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	padding('abc', 8, 'xy', 0)	'abcxyxyx'
2	padding('abcdef', 4, 'xy', 0)	'abcd'
3	padding('go', 7, '123', 1)	'go23123'

Notes

• Mirrors Swift’s padding(toLength:withPad:startingAt:) behavior for truncation and pad-fill.

Here's what our current TypeScript equivalent to Swift's String.padding looks like.

TSTypeScript JSJavaScript TSStandalone TS JSStandalone JS

Copy code

export function padding(str: string, toLength: number, withPad: string = ' ', startingAt: number = 0): string {
  //  discuss at: https://locutus.io/swift/String/padding/
  // original by: Kevin van Zonneveld (https://kvz.io)
  //      note 1: Mirrors Swift's padding(toLength:withPad:startingAt:) behavior for truncation and pad-fill.
  //   example 1: padding('abc', 8, 'xy', 0)
  //   returns 1: 'abcxyxyx'
  //   example 2: padding('abcdef', 4, 'xy', 0)
  //   returns 2: 'abcd'
  //   example 3: padding('go', 7, '123', 1)
  //   returns 3: 'go23123'

  const source = String(str)
  const targetLength = Math.trunc(Number(toLength))
  const pad = String(withPad)

  if (!Number.isFinite(targetLength) || targetLength <= 0) {
    return ''
  }
  if (source.length >= targetLength) {
    return source.slice(0, targetLength)
  }
  if (pad.length === 0) {
    return source
  }

  const start = Math.max(0, Math.trunc(Number(startingAt))) % pad.length
  const needed = targetLength - source.length

  let repeated = ''
  while (repeated.length < needed + start) {
    repeated += pad
  }

  return source + repeated.slice(start, start + needed)
}

export function padding(str, toLength, withPad = ' ', startingAt = 0) {
  //  discuss at: https://locutus.io/swift/String/padding/
  // original by: Kevin van Zonneveld (https://kvz.io)
  //      note 1: Mirrors Swift's padding(toLength:withPad:startingAt:) behavior for truncation and pad-fill.
  //   example 1: padding('abc', 8, 'xy', 0)
  //   returns 1: 'abcxyxyx'
  //   example 2: padding('abcdef', 4, 'xy', 0)
  //   returns 2: 'abcd'
  //   example 3: padding('go', 7, '123', 1)
  //   returns 3: 'go23123'

  const source = String(str)
  const targetLength = Math.trunc(Number(toLength))
  const pad = String(withPad)

  if (!Number.isFinite(targetLength) || targetLength <= 0) {
    return ''
  }
  if (source.length >= targetLength) {
    return source.slice(0, targetLength)
  }
  if (pad.length === 0) {
    return source
  }

  const start = Math.max(0, Math.trunc(Number(startingAt))) % pad.length
  const needed = targetLength - source.length

  let repeated = ''
  while (repeated.length < needed + start) {
    repeated += pad
  }

  return source + repeated.slice(start, start + needed)
}

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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