A function to test how long it takes to do 15,000 function calls and array assignments by index in JavaScript

testSpeed.js

function testSpeed() {
  const stack = [];

  function assignValue(index) {
    stack[index] = 'bob ' + index;
  }

  const t0 = performance.now()
  for (let ix = 0; ix < 15000; ix++) {
    assignValue(ix);
  }
  const t1 = performance.now()

  return [stack, t1 - t0];
}

Tested on Chrome 71 and a Dell Precision 5510 (i7 dual-core CPU) = 4ms.