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Explain JS event loop and related concepts | Rippling Interview Question

Q: Explain JS event loop and related concepts

This question evaluates understanding of core frontend JavaScript concepts including the event loop and task scheduling, differences in variable scoping and hoisting (var/let/const), 'this' binding semantics, and HTML/CSS layout skills for constructing a 2x2 grid, falling under Software Engineering Fundamentals.

You are interviewing for a frontend JavaScript position. Answer the following conceptual and practical questions.

1. JavaScript Event Loop, Microtasks, and Macrotasks

Explain how the JavaScript event loop works in a browser environment.

What is the call stack ?
What is the event loop ?
What are macrotasks (also called tasks)? Give common examples (e.g., setTimeout , setInterval , DOM events, etc.).
What are microtasks ? Give common examples (e.g., Promise.then , MutationObserver , etc.).
In what order are microtasks and macrotasks executed relative to each other and to rendering/painting? Describe a typical sequence for one event loop tick.

2. Difference Between `var` and `let`

Explain the differences between var and let in JavaScript, focusing on:

Scope (function scope vs block scope).
Hoisting behavior.
The "temporal dead zone".
Re-declaration rules.
How they behave when declared in the global scope (relationship to the global object).

Optionally, briefly contrast const with let as well.

3. `this` Binding in JavaScript

Explain how the value of this is determined in the following contexts:

In the global scope.
Inside a regular function call (not attached to an object).
Inside a method call on an object.
Inside an event handler in the browser.
Inside an arrow function.

Then describe at least three different ways to explicitly control or fix the value of this (for example, with bind, call, apply, storing this in another variable, using arrow functions, etc.), and give short code examples.

4. Draw a 2x2 Square Grid with HTML and CSS

Using only HTML and CSS (no JavaScript), write markup and styles to draw a square that is divided into four equal smaller squares (a 2x2 grid), each with visible borders.

Assume the outer square should be, for example, 200px by 200px .
The 4 inner squares should be equal in size and fill the outer square exactly.
Use any reasonable CSS layout technique (e.g., flexbox, CSS grid, or percentage-based widths/heights).

Provide the HTML and CSS code required.

You are interviewing for a frontend JavaScript position. Answer the following conceptual and practical questions.

1. JavaScript Event Loop, Microtasks, and Macrotasks

Explain how the JavaScript event loop works in a browser environment.

What is the call stack ?
What is the event loop ?
What are macrotasks (also called tasks)? Give common examples (e.g., setTimeout , setInterval , DOM events, etc.).
What are microtasks ? Give common examples (e.g., Promise.then , MutationObserver , etc.).
In what order are microtasks and macrotasks executed relative to each other and to rendering/painting? Describe a typical sequence for one event loop tick.

2. Difference Between `var` and `let`

Explain the differences between var and let in JavaScript, focusing on:

Scope (function scope vs block scope).
Hoisting behavior.
The "temporal dead zone".
Re-declaration rules.
How they behave when declared in the global scope (relationship to the global object).

Optionally, briefly contrast const with let as well.

3. `this` Binding in JavaScript

Explain how the value of this is determined in the following contexts:

In the global scope.
Inside a regular function call (not attached to an object).
Inside a method call on an object.
Inside an event handler in the browser.
Inside an arrow function.

4. Draw a 2x2 Square Grid with HTML and CSS

Using only HTML and CSS (no JavaScript), write markup and styles to draw a square that is divided into four equal smaller squares (a 2x2 grid), each with visible borders.

Assume the outer square should be, for example, 200px by 200px .
The 4 inner squares should be equal in size and fill the outer square exactly.
Use any reasonable CSS layout technique (e.g., flexbox, CSS grid, or percentage-based widths/heights).

Provide the HTML and CSS code required.

Explain JS event loop and related concepts

Quick Overview

1. JavaScript Event Loop, Microtasks, and Macrotasks

2. Difference Between `var` and `let`

3. `this` Binding in JavaScript

4. Draw a 2x2 Square Grid with HTML and CSS

Solution

Submit Your Answer

Explain JS event loop and related concepts

Quick Overview

1. JavaScript Event Loop, Microtasks, and Macrotasks

2. Difference Between `var` and `let`

3. `this` Binding in JavaScript

4. Draw a 2x2 Square Grid with HTML and CSS

Solution

Submit Your Answer

Explain JS event loop and related concepts

Quick Overview