7 Tricky JavaScript Interview Questions

js-interview.md

7 Tricky JavaScript Interview Questions

1. Accidental Global

function foo() {
  let a;
  window.b = 0;
  a = window.b;
  a++;
  return a;
}

foo();
typeof a;        // => 'undefined'
typeof window.b; // => 'number'

2. Array Length Property

const clothes = ['jacket', 't-shirt'];
clothes.length = 0;

clothes[0]; // => undefined since cloths is now empty

3. Eagle Eye Test

const length = 4;
const numbers = [];
var i;
for (i = 0; i < length; i++) {
  // does nothing
}
{ 
  // a simple block
  numbers.push(i + 1);
}

numbers; // => [5]

4. Automatic Semicolon Insertion

const length = 4;
const numbers = [];
var i;
for (i = 0; i < length; i++) {
  // does nothing
}
{ 
  // a simple block
  numbers.push(i + 1);
}

numbers; // => [5]

5. Tricky Closure

let i;
for (i = 0; i < 3; i++) {
  const log = () => {
    console.log(i); // 3 3 3
  }
  setTimeout(log, 100);
}

6. Floating Point Math

0.1 + 0.2 === 0.3 // => false

The sum of 0.1 and 0.2 numbers is not exactly 0.3, but slightly above 0.3. Due to how floating point numbers are encoded in binary, operations like addition of floating point numbers are subject to rounding errors.

7. Hoisting

myVar;   // => undefined
myConst; // => ReferenceError

var myVar = 'value';
const myConst = 3.14;