array.ts

class MyArray extends Array {}

slice.js

const len = 10000000;
let str = '';
for (let i = 0; i < len; i++) {
  str += 97 + i % 26;
}

function checkArray(input) {
  let i = -1;
  const l = input.length;
  const array = [];
  while (++i < l) {
    array.push(input[i]);
  }
  array.slice(0);
}

function check(title) {
  const start = Date.now();
  checkArray(str);
  const end = Date.now() - start;
  console.log(`${title}: ${end}ms`);
}

// Typescript generates this function
function __extends(d, b) {
  Object.setPrototypeOf(d, b);
  function __() { this.constructor = d; }
  d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
}

function MyArray() {
}

check('before extend');

__extends(MyArray, Array);

check('after extend');

// Node v8.9.4
// before: 1946
// after: 4277

// Chrome v63.0.3239.132
// before: 1861
// after: 4844

// Firefox 57.0.4
// before: 1338 
// after: 1046