Padding Strings in JavaScript: padLeft & padRight

Here are padLeft and padRight implemented as simple JavaScript functions:

padLeft

function padLeft(str, numChars = 4, char = ' ') {
    return (Array.from({ length: numChars }).fill(char).join('') + str).slice(-1 * numChars)
}
padLeft('Max', 8, ' ') // '     Max'
padLeft('1', 4, '0')   // '0001'

padRight

function padRight(str, numChars = 4, char = ' ') {
    return (str + Array.from({ length: numChars }).fill(char).join('')).slice(0, numChars)
}
padRight('Max', 8, ' ') // 'Max     '
padRight('1', 4, '0')   // '1000'

How it works

The padLeft function from above could also be written like this:

function padLeft(str, numChars = 4, char = ' ') {
    // 1. Create an array of `numChars` length (Array.from)
    // 2. Fill it with `char` characters (Array.prototype.fill)
    // 3. Convert it to a string (Array.prototype.join)
    const characters = Array.from({ length: numChars }).fill(char).join('')
    // 4. Concat the characters and the string
    const charsAndString = characters + str
    // 5. Truncate the string from the right so it's at most
    //    `numChars` characters long
    const paddedString = charsAndString.slice(-1 * numChars)
    return paddedString
}