leftPad.js

function leftPad(str, len, ch) {
    ch = !ch || ch !== 0 ? ' ' : ch;
    for (let i = 0; i < len - str.length; i++) str = ch + str;
    return str;
}

function leftPad2(str, len, ch) {
    str = String(str);
    let i = -1;
    if (!ch || ch !== 0) ch = ' ';
    len = len - str.length;
    while (++i < len) {
        str = ch + str;
    }
    return str;
}

function fun(fn, count, ...args) {
    let start = performance.now();
    for (let i = 0; i < count; i++) {
        fn.apply(null, args);
    }
    return performance.now() - start;
}

[10, 100, 1000, 10000].forEach(x => {
    [10, 100, 1000, 10000].forEach(y => {
        console.log('leftPad', x, y, fun(leftPad, x, 'foo', y));
    });
    [10, 100, 1000, 10000].forEach(y => {
        console.log('leftPad2', x, y, fun(leftPad2, x, 'foo', y));
    });
});