JavaScript Interview Coding Questions — 2

Let’s continue to coding questions.

1. This one is about testing hoisting knowledge in JavaScript.

// What will be the console outputs? Why?
callMe1();
callMe2();
function callMe1() {
  console.log('I am here');
}
var callMe2 = function () {
  console.log('I am here too');
};

You will an error for callMe2(). Console log will be below:

I am here
TypeError: callMe2 is not a function

In JavaScript, basically function and variable definitions are moved to top (which is named as hoisting). The callMe2 variable is defined but it is not a function! This is because, hoisting only moves up to declarations not initializations. So, our callMe2 variable is declared but the function is not assigned to it.

You can test it via this link.

2. This one is for testing the knowledge about const, let and var .

// 1. What will be output?
var v;
let l;
const c;
console.log({ v, l, c });

Again you will get an error.

const c;
      ^
SyntaxError: Missing initializer in const declaration

You must assign a value while declaring a const variable!

You can test it via this link.

3. Another question about variable declarations.

// 2. What will be outputs? Try one by one!
console.log('a:', a);
var a = 1;
console.log('b:', b);
let b = 2;
console.log('c:', c);
const c = 3;

If you try them one by one you will see the following console logs:

a: undefined
ReferenceError: b is not defined
ReferenceError: c is not defined

This is because variables which are declared with let or const are not initialized until their definition is evaluated! So it will throw ReferenceError. You can read Temporal Dead Zone part for more detailed information.

You can test it via this link.

My some other articles:

1. JavaScript Interview Coding Questions — 1
2. Chrome DevTool Tips