core-js/internals/string-pad.js

// https://github.com/tc39/proposal-string-pad-start-end
var toLength = require('../internals/to-length');
var repeat = require('../internals/string-repeat');
var requireObjectCoercible = require('../internals/require-object-coercible');

var ceil = Math.ceil;

// `String.prototype.{ padStart, padEnd }` methods implementation
var createMethod = function (IS_END) {
  return function ($this, maxLength, fillString) {
    var S = String(requireObjectCoercible($this));
    var stringLength = S.length;
    var fillStr = fillString === undefined ? ' ' : String(fillString);
    var intMaxLength = toLength(maxLength);
    var fillLen, stringFiller;
    if (intMaxLength <= stringLength || fillStr == '') return S;
    fillLen = intMaxLength - stringLength;
    stringFiller = repeat.call(fillStr, ceil(fillLen / fillStr.length));
    if (stringFiller.length > fillLen) stringFiller = stringFiller.slice(0, fillLen);
    return IS_END ? S + stringFiller : stringFiller + S;
  };
};

module.exports = {
  // `String.prototype.padStart` method
  // https://tc39.github.io/ecma262/#sec-string.prototype.padstart
  start: createMethod(false),
  // `String.prototype.padEnd` method
  // https://tc39.github.io/ecma262/#sec-string.prototype.padend
  end: createMethod(true)
};