locutus/esm/swift/String/padding.js

Locutus other languages' standard libraries to JavaScript for fun and educational purposes

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);
}