---
title: "Lessons learned from JavaScript quizzes"
description:
  "Recently I tried to answer some JavaScript quizzes and in the process learned
  a bit. This blog discusses things that I learned."
canonical_url: "https://www.bigbinary.com/blog/lessons-learned-from-javascript-quizzes"
markdown_url: "https://www.bigbinary.com/blog/lessons-learned-from-javascript-quizzes.md"
---

# Lessons learned from JavaScript quizzes

Recently I tried to answer some JavaScript quizzes and in the process learned a
bit. This blog discusses things that I learned.

- Author: Neeraj Singh
- Published: March 15, 2010
- Categories: JavaScript

Nicholas answered three JavaScript quizzes in his blog. I am not interested in
quiz like the one given below

```javascript
var num1 = 5,
  num2 = 10,
  result = num1++ + num2;
```

However some of the questions helped me learn a few things.

## Questions from quiz

Recently there was a
[quiz](http://www.nczonline.net/blog/2010/02/23/answering-soshnikovs-quiz) out.

This was question #5 in the original blog. I have modified the quiz a little bit
to suit my needs.

```javascript
var x = 10;
var foo = {
  x: 20,
  bar: function () {
    var x = 30;
    return this.x;
  },
};

// 1
console.log(foo.bar());

// 2
console.log(foo.bar());

// 3
console.log(foo.bar.call());
```

I got the first two answers wrong. In JavaScript a variable and a property are
two different things. When `this.xyx` is invoked then JavaScript engine is
looking for property called `xyz`.

```javascript
var bar = function () {
  var x = 30;
  return this.x;
};
console.log(bar()); //=> undefined
```

In the above case output is `undefined`. This is because `this` refers to a
property named `x` and since no such property was found `undefined` is the
answer.

```javascript
var foo = {
  x: 20,
  bar: function () {
    return x;
  },
};
console.log(foo.bar());
```

Above code causes `ReferenceError` because `x is not defined`. Same theory
applies here. In this case `x` is a variable and since no such variable was
found code failed.

Coming back to the third part of the original question. This one uses call.

```javascript
console.log(foo.bar.call());
```

First argument of `call` or `apply` method determines what `this` would be
inside the function. If no argument is passed then JavaScript engine assumes
that `this` would be global scope which translates to `this` being `window`.
Hence the answer is `10` in this case.

## Questions from another quiz

There was another
[quiz](http://www.nczonline.net/blog/2010/02/18/my-javascript-quiz-answers) .

In the original blog this is question #2.

```javascript
var x = 5,
  o = {
    x: 10,
    doIt: function doIt() {
      var x = 20;
      setTimeout(function () {
        alert(this.x);
      }, 10);
    },
  };
o.doIt();
```

The key thing to remember here is that
`All functions passed into setTimeout() are executed in the global scope` .

In the original blog this is question #5.

```javascript
var o = {
    x: 8,

    valueOf: function () {
      return this.x + 2;
    },
    toString: function () {
      return this.x.toString();
    },
  },
  result = o < "9";

alert(o);
```

The thing to remember here is that when comparison is done then `valueOf` method
is called on the object.

## Questions from [quiz](http://www.nczonline.net/blog/2010/01/26/answering-baranovskiys-javascript-quiz)

This is question #1 in the original blog.

```javascript
if (!("a" in window)) {
  var a = 1;
}
alert(a);
```

I knew that all the variable declarations are hoisted up but somehow failed to
apply that logic here. Please see the original blog for a detailed answer.

This is question #5 in the original blog.

```javascript
function a() {
  alert(this);
}
a.call(null);
```

I knew that if nothing is passed to `call` method then `this` becomes global but
did not know that if `null` is passed then also `this` becomes global.

## Links

- [Human page](https://www.bigbinary.com/blog/lessons-learned-from-javascript-quizzes)