Interview Question

interview.md

Javascript

Question 1

In what order will the numbers 1-4 be logged to the console when the code below is executed? Why?

(function() {
    console.log(1); 
    setTimeout(function(){console.log(2)}, 1000); 
    setTimeout(function(){console.log(3)}, 0); 
    console.log(4);
})();

The values will be logged in the following order:

1
4
3
2

Let’s first explain the parts of this that are presumably more obvious:

1 and 4 are displayed first since they are logged by simple calls to console.log() without any delay 2 is displayed after 3 because 2 is being logged after a delay of 1000 msecs (i.e., 1 second) whereas 3 is being logged after a delay of 0 msecs.

Question 2

What will be the output when the following code is executed?

console.log(false == '0')
console.log(false === '0')

The code will output:

true
false

In JavaScript, there are two sets of equality operators. The triple-equal operator === behaves like any traditional equality operator would: evaluates to true if the two expressions on either of its sides have the same type and the same value. The double-equal operator, however, tries to coerce the values before comparing them. It is therefore generally good practice to use the === rather than ==. The same holds true for !== vs !=.

Question 3

What will be the output of this code?

var x = 21;
var number = function () {
    console.log(x);
    var x = 20;
};
number();

The code will output: undefined

It’s because JavaScript initialization is not hoisted. (Why doesn’t it show the global value of 21? The reason is that when the function is executed, it checks that there’s a local x variable present but doesn’t yet declare it, so it won’t look for global one.)