because of Primagen https://www.youtube.com/watch?v=NmHUjxKpD90

leftpad_time.js

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

function leftPadP(str,len,ch) {
    return new Array(len-str.length).fill(!ch&&ch!=0?' ':ch).join("")+str
}

function leftPadP2(str,len,ch) {
    return new Array(len-str.length).join(!ch&&ch!=0?' ':ch)+str
}

new Array(42).join(' ')


function leftPadA(str, len, ch) {
    s = "";
    var i=-1;
    if (!ch && ch != 0) ch=' ';
    len -= str.length;
    while (++i<len) {
        s += ch;
    }
    return s + String(str);
}

function leftPadA2(str, len, ch)
{
    return str.padStart(len, !ch&&ch!=0?' ':ch);
}

function run(fn, count, ...args) {
    const start = performance.now();
    for (i=0;i<count;++i) {
        ret = fn.apply(null,args);
        //console.log(ret);
    }
    return performance.now()-start;
}

console.log("loops","spaces","leftPad(orig)","Prime  ", "Prime2   ", "Aaron   ","padStart");

[100,1000].forEach(x=> {
    [100,1000,10000,100000].forEach(y=> {
        console.log(x.toString().padStart(5),y.toString().padStart(6),
            run(leftPad,x,'foo',y).toFixed(3).padStart(9),
            run(leftPadP,x,'foo',y).toFixed(3).padStart(9),
            run(leftPadP2,x,'foo',y).toFixed(3).padStart(9),
            run(leftPadA,x,'foo',y).toFixed(3).padStart(9), 
            run(leftPadA2,x,'foo',y).toFixed(3).padStart(9), 
            );
    });
});

nodejs v19.7.0, AMD 5625U laptop
node .\leftPad_time.js
loops spaces leftPad(orig) Prime   Prime2    Aaron    padStart
  100    100     1.239     0.396     0.615     0.621     0.237
  100   1000     1.123     1.482     0.528     1.348     0.015
  100  10000    11.119    17.642     5.044    16.160     0.016
  100 100000   111.831   319.022    75.449   223.784     0.020
 1000    100     2.633     1.767     0.762     1.225     0.112
 1000   1000     4.382    17.806     6.985    13.442     0.240
 1000  10000    47.784   172.958    60.518   132.750     0.118
 1000 100000  1298.322  3350.577   790.705  2402.140     0.121