Menu Close

9 Common JavaScript Interview Questions

9 Common JavaScript Interview Questions

Whether you like them or not, tricky questions are still asked by interviewers in the wild.

The reason is, these questions can tell a lot about your core understanding of the language, and therefore if you are a fit for the job.

Common concepts addressed in these questions include:

  • Hoisting
  • Closure
  • Scope
  • Value vs Reference type
  • Prototype inheritance

Today we will kill two birds with one stone.

Prepare to ace your next interview and brush up on the core concepts, at once.



Why typeof f is undefined?

var y = 1;
if (function f() {}) {
  y += typeof f;
}

console.log(y); // 1undefined
Enter fullscreen mode

Exit fullscreen mode

Explanation

  • The condition statement if(function f() {}) returns function f() {} which is truthy, so the code executes inside the if statement.
  • typeof f returns undefined because the function f(){} was never declared outside the if statement, so it doesn’t “exist” outside the if brackets if (f(){}) { It doesn't exist here }

This is how we would “fix” this code snippet:

var y = 1;

function f() {}; //declare function outside if brackets

if (f) { //f exists so we enter the if block
  y += typeof f; //here typeof f is function
}

console.log(y); // 1function
Enter fullscreen mode

Exit fullscreen mode



Write an example of closure

function createFunction(msg) {
    return function(name) {
        return msg + name;
    }
}

let myFunc = createFunction("Hey ");
console.log(myFunc("Dude")); // Hey Dude
Enter fullscreen mode

Exit fullscreen mode

Explanation

  • The first function (createFunction) returns an anonymous function
  • The anonymous function returns the parameter msg from the outer function(createFunction()) + name from itself.

When we declare the variable javascript let myFunc = createFunction("Hey ") the variable myFunc holds a reference to the anonymous function returned by the outer

What is interesting is that when you call javascript myFunc("Dude") (passing just the ‘name’ parameter) it still “remembers” the value of msg that was passed in when createFunction() was executed.

The ability to access variables from the outer ‘scope’ where the function was created is one of the definitions of closure.

Learn more about closures here



Write a multiply() function that can be called like multiply(2)(5)(10) and returns 100

This is similar to the example above, the difference being that we are retuning an additional function and invoking all of them straight away(no using variables for holding references)

function multiply(x) {
    return function(y){
        return function(z) {
          return x*y*z;
        };
    }
}

multiply(2)(5)(10) //100
Enter fullscreen mode

Exit fullscreen mode

By invoking multiply like

javascript multiply(2)(5)(10)

We are invoking the returned functions one after the other.

We could use intermediate variables to make it clearer. Let’s have a look:

let func1 = multiply(2); // x is 2
console.log(func1) // function(y) { return function(z) { return x*y*z } }

let func2 = func1(5); // y is 5
console.log(func1) // function(z) { return x*y*z }

func2(10); // z is 10
// finally has all 3 values and returns their product.
Enter fullscreen mode

Exit fullscreen mode

Learn more aboute Nested Functionshere



delete operator on a local variable

What would be the output of the following code?

let output = (function(x) {
    delete x;
    return x;
})(0);
Enter fullscreen mode

Exit fullscreen mode

*The delete operator is meant to be used to delete properties of objects, and not on value types(number in this case).

This would work:

let obj = { name: 'Gus', age: 32 }
delete obj.age;

console.log(obj) // { name: 'Gus' }
Enter fullscreen mode

Exit fullscreen mode



delete operator on Object

const Person = {
    name: 'Gus',
    age: 32,
}

const person1 = Object.create(Person);
delete person1.age

console.log(person1.Age); // 32
Enter fullscreen mode

Exit fullscreen mode

  • person1 is created with its prototype set to the Person object.
  • When the instance ’emp1′ has it’s ‘name’ property deleted, we can still access the ‘name’ property of the prototype object(Person)
  • That is why it seems like it didn’t work.

This is a big topic, you can learn more here



delete operator on Array

What would be the result of logging arr.length ?

  let arr = ["a", "b", "c", "d"];
  delete arr[2];

  arr.length // 4
Enter fullscreen mode

Exit fullscreen mode

  • When used on arrays, the delete operator sets the delete element to ’empty’ but does not remove it from the array, not changing the length of the array.



What will be the value of both console.log’s?

var favouriteAnime = "Dragon Ball";
(function() {
    console.log(favouriteAnime);
    var favouriteAnime = "Naruto";
    console.log(favouriteAnime);
})();

// undefined 
// Naruto
Enter fullscreen mode

Exit fullscreen mode

Here is how the compiler interpreted this code:

var favouriteAnime; // declared and initialized with undefined
(function() {
    console.log(favouriteAnime);
    var favouriteAnime = "Naruto";
    console.log(favouriteAnime);
})();

favouriteAnime = "Dragon Ball";

// undefined 
// Naruto
Enter fullscreen mode

Exit fullscreen mode

A few things to keep in mind when declaring JavaScript functions and variables.

  • Variable assignment(myVar = 5) takes precedence over function declaration(function func(){})
  • Function declarations(function func(){}) take precedence over variable declarations(var myVar;) *let or const
  • Function declarations(function func(){}) are hoisted over variable declarations(var myVar;) but not over variable assignments(myVar = 5;).

As a best practice, you should always declare your functions before invoking them.

Learn more about hoisting here



How would you check if a number is an integer?

A very simple way to check if a number is a decimal or integer is to see if there is a remainder left when you divide by 1.

function isInt(num) {
  return num % 1 === 0;
}

console.log(isInt(4)); // true
console.log(isInt(12.2)); // false
console.log(isInt(0.3)); // false
Enter fullscreen mode

Exit fullscreen mode



Conclusion

Remember to hit f12 and try out the examples for yourself.

This tiny action will help you remember what you learned for much longer.

This is a recap of what we have seen today:

  • Why typeof f is undefined?
  • Write an example of closure
  • Write a multiply() function that can be called like multiply(2)(5)(10) and returns 100
  • Delete operator on a local variable
  • Delete operator on Object
  • Delete operator on Array
  • What will be the value of both console.log’s?
  • How would you check if a number is an integer?

Thanks for reading!

If you like this article:

*Leave a comment below(You can just say hi!)
*Follow me on Twitter @theguspear.

Catch you later my dude,

Gus.

View Source
Posted in JavaScript