polyfill.js

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/findIndex#Specifications
// https://tc39.github.io/ecma262/#sec-array.prototype.findIndex
if (!Array.prototype.findIndex) {
  console.warn('define polyfill for Array.prototype.findIndex');
  Object.defineProperty(Array.prototype, 'findIndex', {
    value: function(predicate) {
      // 1. Let O be ? ToObject(this value).
      if (this == null) {
        throw new TypeError('"this" is null or not defined');
      }
      var o = Object(this);
      // 2. Let len be ? ToLength(? Get(O, "length")).
      var len = o.length >>> 0;
      // 3. If IsCallable(predicate) is false, throw a TypeError exception.
      if (typeof predicate !== 'function') {
        throw new TypeError('predicate must be a function');
      }
      // 4. If thisArg was supplied, let T be thisArg; else let T be undefined.
      var thisArg = arguments[1];
      // 5. Let k be 0.
      var k = 0;
      // 6. Repeat, while k < len
      while (k < len) {
        // a. Let Pk be ! ToString(k).
        // b. Let kValue be ? Get(O, Pk).
        // c. Let testResult be ToBoolean(? Call(predicate, T, « kValue, k, O »)).
        // d. If testResult is true, return k.
        var kValue = o[k];
        if (predicate.call(thisArg, kValue, k, o)) {
          return k;
        }
        // e. Increase k by 1.
        k++;
      }
      // 7. Return -1.
      return -1;
    }
  });
}

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/fill
if (!Array.prototype.fill) {
	console.warn('define polyfill for Array.prototype.fill');
  Object.defineProperty(Array.prototype, 'fill', {
    value: function(value) {

      // Steps 1-2.
      if (this == null) {
        throw new TypeError('this is null or not defined');
      }

      var O = Object(this);

      // Steps 3-5.
      var len = O.length >>> 0;

      // Steps 6-7.
      var start = arguments[1];
      var relativeStart = start >> 0;

      // Step 8.
      var k = relativeStart < 0 ?
        Math.max(len + relativeStart, 0) :
        Math.min(relativeStart, len);

      // Steps 9-10.
      var end = arguments[2];
      var relativeEnd = end === undefined ?
        len : end >> 0;

      // Step 11.
      var final = relativeEnd < 0 ?
        Math.max(len + relativeEnd, 0) :
        Math.min(relativeEnd, len);

      // Step 12.
      while (k < final) {
        O[k] = value;
        k++;
      }

      // Step 13.
      return O;
    }
  });
}

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/startsWith
if (!String.prototype.startsWith) {
    console.warn('define polyfill for String.prototype.startsWith');
    String.prototype.startsWith = function (searchString, position) {
      position = position || 0;
      return this.substr(position, searchString.length) === searchString;
  };
}