polyfill.js

(function () {
  /**
   * Array.prototype.find()
   * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/find
   */
  if (!Array.prototype.find) {
    Object.defineProperty(Array.prototype, 'find', {
      value: function (predicate) {
        'use strict';
        if (this == null) {
          throw new TypeError('Array.prototype.find called on null or undefined');
        }
        if (typeof predicate !== 'function') {
          throw new TypeError('predicate must be a function');
        }
        var list = Object(this);
        var length = list.length >>> 0;
        var thisArg = arguments[1];
        var value;

        for (var i = 0; i < length; i++) {
          value = list[i];
          if (predicate.call(thisArg, value, i, list)) {
            return value;
          }
        }
        return undefined;
      }
    });
  }
  /**
   * Array.prototype.findIndex()
   * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/findIndex
   */
  if (!Array.prototype.findIndex) {
    Object.defineProperty(Array.prototype, 'findIndex', {
      value: function (predicate) {
        'use strict';
        if (this == null) {
          throw new TypeError('Array.prototype.findIndex called on null or undefined');
        }
        if (typeof predicate !== 'function') {
          throw new TypeError('predicate must be a function');
        }
        var list = Object(this);
        var length = list.length >>> 0;
        var thisArg = arguments[1];
        var value;

        for (var i = 0; i < length; i++) {
          value = list[i];
          if (predicate.call(thisArg, value, i, list)) {
            return i;
          }
        }
        return -1;
      },
      enumerable: false,
      configurable: false,
      writable: false
    });
  }
}());