JavaScript is one of the most important programming languages in modern web development. Whether you are preparing for frontend, backend, or full-stack roles, interviewers frequently test your understanding of core JavaScript concepts.
In this post on DEV Community, we will explore the top 5 most commonly asked JavaScript interview questions with simple explanations and examples.
✅ 1. What is Closure in JavaScript?
✔ Answer:
A closure is created when a function remembers and can access variables from its outer function even after the outer function has finished execution.
Closures help in:
Data privacy
Maintaining state
Creating function factories
Example:
function outer() {
let count = 0;
return function inner() {
count++;
console.log(count);
};
}
const counter = outer();
counter();
counter();
✔ Explanation:
The inner function remembers the count variable even after the outer function is executed. This is called a closure.
✅ 2. What is the Difference Between var, let, and const?
✔ Answer:
These keywords are used to declare variables but behave differently.
var
Function scoped
Can be redeclared
Older way of declaring variables
let
Block scoped
Can be updated but not redeclared
const
Block scoped
Cannot be reassigned
Used for constant values
Example:
let age = 25;
age = 30;
const name = "John";
✔ Interview Tip:
Most modern projects prefer let and const over var.
✅ 3. What is the Event Loop in JavaScript?
✔ Answer:
JavaScript is a single-threaded language, meaning it executes one task at a time. The event loop manages asynchronous operations like timers, API calls, and promises.
It ensures:
Non-blocking execution
Smooth handling of async tasks
Better performance
Example:
console.log("Start");
setTimeout(() => {
console.log("Async Task");
}, 0);
console.log("End");
✔ Output:
Start
End
Async Task
The event loop pushes asynchronous tasks into the queue and executes them after the main execution stack is cleared.
✅ 4. What is the Difference Between == and === ?
✔ Answer:
Both are comparison operators but behave differently.
== (Loose Equality)
Compares values
Performs type conversion
=== (Strict Equality)
Compares value and data type
More reliable and recommended
Example:
console.log(5 == "5"); // true
console.log(5 === "5"); // false
✔ Interview Tip:
Always prefer === to avoid unexpected results.
✅ 5. What are Promises and Async/Await?
✔ Answer:
Promises and Async/Await help handle asynchronous operations in JavaScript.
Promise
A promise represents a value that may be available now, later, or never.
Example:
let promise = new Promise((resolve, reject) => {
resolve("Success");
});
Async / Await
Async/Await makes asynchronous code easier to read and write.
Example:
async function fetchData() {
let response = await fetch("https://api.example.com");
console.log("Data received");
}
Exercise: https://www.quipoin.com/exercise/java-script
Interview: https://www.quipoin.com/interview/java-script
MCQs: https://www.quipoin.com/practice-mcqs/java-script
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