Interview Q&A / JavaScript

1. What is a closure in JavaScript?

   A closure is a function that remembers the variables from the place where it was created, even after that outer function has finished running.

   BeginnerIn-depth2 min read
2. How does the this keyword work in JavaScript?

   this is decided by how a function is called, not where it is written. A method call obj.fn() sets this to obj; new sets it to the new object; call, apply and bind set it explicitly; a plain call gives undefined in strict mode. Arrow functions have no own this and use the surrounding one.

   IntermediateIn-depth3 min read
3. Is JavaScript pass by value or pass by reference?

   JavaScript is always pass by value. For objects, the value passed is a reference, so a function can change the object's contents but cannot make the caller's variable point to a different object.

   BeginnerQuick answer1 min read
4. What is an IIFE and why would you use one?

   An IIFE (immediately invoked function expression) is a function that is defined and called in one go, like (function () { ... })(). It creates a private scope so variables inside do not leak out.

   BeginnerQuick answer1 min read
5. What is the difference between a function declaration and a function expression?

   A declaration is hoisted with its body, so you can call it before the line it appears on. An expression assigns a function to a variable, and the variable follows var, let or const rules, so calling it early fails.

   BeginnerQuick answer1 min read
6. What is the difference between call, apply and bind?

   All three set this for a function. call runs it now with arguments listed one by one, apply runs it now with an array of arguments, and bind returns a new function with this fixed, to be run later.

   BeginnerQuick answer1 min read