Workaround for not jet implemented String.padStart/padEnd

Almost there, but this blows up of the len is smaller than the length of the string without padding. A small modification solves it ((len-sbl>0)):

var len = 6;   
var sb = parseInt(1023).toString(2);
var sbl = sb.length;
var sbFixedLenStart = ('0').repeat((len-sbl>0)?(len-sbl):0) + sb;
var sbFixedLenEnd = sb + ('0').repeat((len-sbl>0)?(len-sbl):0);
console.log(sbFixedLenStart);
console.log(sbFixedLenEnd);

MDN usually has polyfills or in this case, a link to polyfills: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/padStart
Altho might need some tweaks to better fit Espruino's constraints...