Top 10 Javascript Interview Questions 2024
JavaScript, the dynamic scripting language that powers the interactive elements of virtually every modern website, has become an integral skill for developers. Aspiring and seasoned developers alike often find themselves navigating through JavaScript-centric interviews, where a thorough understanding of the language can make all the difference. In this article, we delve into the top 10 most asked JavaScript interview questions, uncovering the intricacies of the language and offering insights that will not only sharpen your interview skills but also deepen your comprehension of JavaScript's nuances. Whether you're a seasoned developer preparing for a career move or a budding coder venturing into the vast world of JavaScript, these questions will serve as a compass, guiding you through the essential concepts and scenarios that frequently surface during interviews. So, let's embark on this journey to unravel the mysteries of JavaScript and equip ourselves with the knowledge needed to ace those critical interviews.
- What is a prototype chain?
Prototype chaining is used to build new types of objects based on existing ones. It is similar to inheritance in a class based language.
The prototype on object instance is available through Object.getPrototypeOf(object) or _proto_ property whereas prototype on constructors function is available through Object.prototype.
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What is the difference between == and === operators?
JavaScript provides both strict(===, !==) and type-converting(==, !=) equality comparison. The strict operators take type of variable in consideration, while non-strict operators make type correction/conversion based upon values of variables. The strict operators follow the below conditions for different types,
- Two strings are strictly equal when they have the same sequence of characters, same length, and same characters in corresponding positions.
- Two numbers are strictly equal when they are numerically equal, i.e., having the same number value. There are two special cases in this,
- NaN is not equal to anything, including NaN.
- Positive and negative zeros are equal to one another.
- Two Boolean operands are strictly equal if both are true or both are false.
- Two objects are strictly equal if they refer to the same Object.
- Null and Undefined types are not equal with ===, but equal with ==, i.e, null===undefined --> false, but null==undefined --> true
Some of the example which covers the above cases:
0 == false // true 0 === false // false 1 == "1" // true 1 === "1" // false null == undefined // true null === undefined // false '0' == false // true '0' === false // false []==[] or []===[] //false, refer different objects in memory {}=={} or {}==={} //false, refer different objects in memory -
What are lambda or arrow functions?
An arrow function is a shorter syntax for a function expression and does not have its own this, arguments, super, or new.target. These functions are best suited for non-method functions, and they cannot be used as constructors.
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What is a higher order function?
A higher-order function is a function that accepts another function as an argument or returns a function as a return value or both.
const firstOrderFunc = () => console.log("Hello, I am a First order function"); const higherOrder = (ReturnFirstOrderFunc) => ReturnFirstOrderFunc(); higherOrder(firstOrderFunc); -
What is an IIFE (Immediately Invoked Function Expression)?
IIFE (Immediately Invoked Function Expression) is a JavaScript function that runs as soon as it is defined. The signature of it would be as below,
(function () { // logic here })();The primary reason to use an IIFE is to obtain data privacy because any variables declared within the IIFE cannot be accessed by the outside world. i.e, If you try to access variables from the IIFE then it throws an error as below,
(function () { var message = "IIFE"; console.log(message); })(); console.log(message); //Error: message is not defined -
What is memoization?
Memoization is a functional programming technique which attempts to increase a function’s performance by caching its previously computed results. Each time a memoized function is called, its parameters are used to index the cache. If the data is present, then it can be returned, without executing the entire function. Otherwise the function is executed and then the result is added to the cache. Let's take an example of adding function with memoization,
const memoizAddition = () => { let cache = {}; return (value) => { if (value in cache) { console.log("Fetching from cache"); return cache[value]; // Here, cache.value cannot be used as property name starts with the number which is not a valid JavaScript identifier. Hence, can only be accessed using the square bracket notation. } else { console.log("Calculating result"); let result = value + 20; cache[value] = result; return result; } }; }; // returned function from memoizAddition const addition = memoizAddition(); console.log(addition(20)); //output: 40 calculated console.log(addition(20)); //output: 40 cached -
What is Hoisting?
Hoisting is a JavaScript mechanism where variables, function declarations and classes are moved to the top of their scope before code execution. Remember that JavaScript only hoists declarations, not initialisation. Let's take a simple example of variable hoisting
console.log(message); //output : undefined var message = "The variable Has been hoisted";The above code looks like as below to the interpreter,
var message; console.log(message); message = "The variable Has been hoisted";In the same fashion, function declarations are hoisted too
message("Good morning"); //Good morning function message(name) { console.log(name); }This hoisting makes functions to be safely used in code before they are declared.
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What are closures?
A closure is the combination of a function and the lexical environment within which that function was declared. i.e, It is an inner function that has access to the outer or enclosing function’s variables. The closure has three scope chains
1. Own scope where variables defined between its curly brackets
2. Outer function’s variables
3. Global variables
Let's take an example of closure concept,
function Welcome(name) {
var greetingInfo = function (message) {
console.log(message + " " + name);
};
return greetingInfo;
}
var myFunction = Welcome("John");
myFunction("Welcome "); //Output: Welcome John
myFunction("Hello Mr."); //output: Hello Mr.John
As per the above code, the inner function(i.e, greetingInfo) has access to the variables in the outer function scope(i.e, Welcome) even after the outer function has returned.
- What is a promise?
A promise is an object that may produce a single value some time in the future with either a resolved value or a reason that it’s not resolved(for example, network error). It will be in one of the 3 possible states: fulfilled, rejected, or pending.
The syntax of Promise creation looks like below,
const promise = new Promise(function (resolve, reject) {
// promise description
});
The usage of a promise would be as below,
const promise = new Promise(
(resolve) => {
setTimeout(() => {
resolve("I'm a Promise!");
}, 5000);
},
(reject) => {}
);
promise.then((value) => console.log(value));
The action flow of a promise will be as below,
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What is a callback hell?
Callback Hell is an anti-pattern with multiple nested callbacks which makes code hard to read and debug when dealing with asynchronous logic. The callback hell looks like below,
async1(function(){ async2(function(){ async3(function(){ async4(function(){ .... }); }); }); });
Mastering JavaScript interview questions opens doors to exciting opportunities. These insights not only enhance your interview performance but deepen your grasp of JavaScript's intricacies. Armed with this knowledge, you're well-prepared to navigate the challenges of interviews and excel in your ongoing journey as a JavaScript developer. Keep coding, stay curious, and embrace the dynamic world of JavaScript. Best of luck in all your endeavors!
