Interviewer: Can “x !== x” Return True in JavaScript？

Preface

Recently I was asked a few strange interview questions. They are different from the regular questions: these interview questions seem very easy, but they test your thorough understanding of JavaScript. How many can you answer correctly?

1. Can “x !== x” return true？

What should be the value of “x” to output “hello world”?

const x = ? // Please fill in the value of "x?
if (x !== x) {
  console.log('hello world')
}

It’s amazing. Is there any value that is not equal to itself? However, there is a value NaN in JavaScript, which is not equal to any value or even itself.

const x = NaN // Please fill in the value of "x?
if (x !== x) {
  console.log('hello world')
}
console.log(NaN === NaN) // false
console.log(x !== x) // true
console.log(Number.isNaN(x)) // true

2. Can (!isNaN(x) && x !== x) return true?

Okay, when we filter out “NaN”, what other value can make a value not equal to itself?

const x = ? // Please fill in the value of "x?
if(!isNaN(x) && x !== x) {
  console.log('hello world')
}

Maybe you know “object. Defineproperty”, which can help us solve this problem.

window.x = 0 // Any value is OK
Object.defineProperty(window, 'x', {
  get () {
    return Math.random()
  }
})
console.log(x) // 0.12259077808826002
console.log(x === x) // false
console.log(x !== x) // true

3. How to make “x === x + 1”?

This question may not be easy, but as long as you know JavaScript, you will know “The Number.MAX_SAFE_INTEGER constant represents the maximum safe integer in JavaScript (2⁵³ — 1).”(From MDN)

const x = ? // Please fill in the value of "x?
if (x === x + 1) {
  console.log('hello world')
}

So we can assign “x” any value greater than “Number.MAX_SAFE_INTEGER”.

const x =  Number.MAX_SAFE_INTEGER + 1// Please fill in the value of "x?
if (x === x + 1) {
  console.log('hello world')
}

4. Can “x > x” be true？

I don’t want to read anymore, what kind of rubbish question is that?

Although it seems unlikely, how can a value be greater than itself? However, we can use the “Symbol.toPrimitive” feature to complete the question.

const x = { // Please fill in the value of "x?
  value: 1,
  [ Symbol.toPrimitive ] () {
    console.log('x', this.value)
    return --this.value
  }
}

if (x > x) {
  console.log('hello world')
}

5. typeof x === ‘undefined’ && x.length > 0 ?

const x = ? // Please fill in the value of "x?
if(typeof x === 'undefined' && x.length > 0) {
  console.log('hello world')
}

I have to admit that JavaScript is an amazing language. Besides undefined itself, what other value can make typeof x === undefined” true?

The answer is document. All An HTMLAllCollection which contains every element in the document (from MDN).

const x = document.all // Please fill in the value of "x?
if(typeof x === 'undefined' && x.length > 0) {
  console.log('hello world')
}

console.log(x)
console.log(typeof x)
console.log(x === undefined)