JavaScript runtime overview - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a JavaScript runtime?

A JavaScript runtime is an environment where JavaScript code runs. It provides tools like memory management, event handling, and APIs to interact with the system or browser.

beginner

Name two popular JavaScript runtimes.

Two popular JavaScript runtimes are the browser (like Chrome's V8 engine) and Node.js, which runs JavaScript outside the browser.

intermediate

What role does the event loop play in the JavaScript runtime?

The event loop manages asynchronous tasks by checking the message queue and executing callbacks when the call stack is empty, allowing non-blocking code execution.

beginner

What is the call stack in a JavaScript runtime?

The call stack keeps track of function calls. It follows last-in, first-out order, running the top function and removing it when done before moving to the next.

intermediate

How does Node.js differ from browser JavaScript runtime?

Node.js provides extra APIs for file system, networking, and more, allowing JavaScript to run on servers, while browsers focus on web page interaction and DOM manipulation.

What does the JavaScript event loop do?

A Manages asynchronous tasks and executes callbacks

B Compiles JavaScript code to machine code

C Stores variables and functions

D Handles user interface rendering

The event loop manages asynchronous tasks by executing callbacks when the call stack is empty.

Which of these is NOT a JavaScript runtime?

A Chrome browser

B Python interpreter

C Node.js

D V8 engine

Python interpreter is not a JavaScript runtime; it runs Python code.

Where does JavaScript run in a browser?

A In the operating system kernel

B In the call stack only

C On the server only

D Inside the JavaScript runtime environment

JavaScript runs inside the browser's runtime environment, which includes the engine and APIs.

What is the call stack used for?

A Managing asynchronous events

B Storing global variables

C Tracking function calls in order

D Rendering HTML elements

The call stack tracks function calls in last-in, first-out order.

Which API is available in Node.js but NOT in browsers?

A File system access

B setTimeout

C DOM manipulation

D console.log

File system access is available in Node.js but not in browsers.

Explain the main components of a JavaScript runtime and how they work together.

Think about how JavaScript runs code and handles tasks that take time.

Describe the differences between running JavaScript in a browser and in Node.js.

Consider what each environment allows JavaScript to do.

Practice

1. What is the main role of the JavaScript runtime environment?

easy

A. To compile JavaScript into machine code before running

B. To design the user interface of a web page

C. To execute JavaScript code and provide necessary tools

D. To store JavaScript files on the server

Solution

1. Step 1: Understand what a runtime environment does

   The runtime environment is where JavaScript code runs and interacts with other parts like APIs and event loop.

2. Step 2: Identify the main role

   Its main role is to execute JavaScript code and provide tools like APIs and event handling.

3. Final Answer:

   To execute JavaScript code and provide necessary tools -> Option C

4. Quick Check:

   Runtime runs code and tools = B [OK]

Hint: Runtime runs code and tools, not UI or storage [OK]

Common Mistakes:

• Confusing runtime with compiler
• Thinking runtime designs UI
• Assuming runtime stores files

2. Which of the following is the correct way to access the global object in a JavaScript runtime?

easy

A. globalThis

B. window

C. document

D. runtime

Solution

1. Step 1: Recall the global object in modern JavaScript

   The global object is accessible via globalThis in all environments.

2. Step 2: Compare options

   window is global in browsers only, document is for DOM, runtime is not a valid object.

3. Final Answer:

   globalThis -> Option A

4. Quick Check:

   Universal global object = globalThis [OK]

Hint: Use globalThis for universal global access [OK]

Common Mistakes:

• Using window in non-browser environments
• Confusing document with global object
• Assuming runtime is a global object

3. What will the following code output in a typical JavaScript runtime?

console.log(typeof setTimeout);

medium

A. "undefined"

B. "function"

C. "object"

D. "number"

Solution

1. Step 1: Understand what setTimeout is

   setTimeout is a timer API function provided by the runtime environment.

2. Step 2: Determine the type of setTimeout

   Since setTimeout is a function, typeof setTimeout returns "function".

3. Final Answer:

   "function" -> Option B

4. Quick Check:

   setTimeout is a function = "function" [OK]

Hint: setTimeout is a function, so typeof returns "function" [OK]

Common Mistakes:

• Thinking setTimeout is undefined
• Confusing setTimeout with a number
• Assuming setTimeout is an object

4. Identify the error in this code snippet related to JavaScript runtime APIs:

setTimeout(() => {
  console.log('Hello');
}, '1000');

medium

A. The delay should be a number, not a string

B. Arrow functions cannot be used with setTimeout

C. console.log is not available in runtime

D. setTimeout requires three arguments

Solution

1. Step 1: Check the setTimeout parameters

   setTimeout expects the delay as a number, but here it is a string '1000'.

2. Step 2: Understand the impact

   Passing a string may cause the delay to be treated as 0 or converted, which is incorrect usage.

3. Final Answer:

   The delay should be a number, not a string -> Option A

4. Quick Check:

   Delay must be number, not string [OK]

Hint: Delay argument must be a number, not a string [OK]

Common Mistakes:

• Using string instead of number for delay
• Thinking arrow functions are invalid here
• Assuming console.log is unavailable

5. You want to run a piece of JavaScript code repeatedly every 2 seconds using the runtime environment. Which code snippet correctly achieves this?

hard

A. repeat(() => console.log('Repeat'), 2000);

B. setTimeout(() => console.log('Repeat'), 2000);

C. while(true) { console.log('Repeat'); }

D. setInterval(() => console.log('Repeat'), 2000);

Solution

1. Step 1: Identify the correct API for repeated execution

   setInterval runs a function repeatedly at specified intervals.

2. Step 2: Compare options

   setTimeout runs once, while loop blocks execution, and repeat is not a valid function.

3. Final Answer:

   setInterval(() => console.log('Repeat'), 2000); -> Option D

4. Quick Check:

   Repeated runs use setInterval [OK]

Hint: Use setInterval for repeated actions every interval [OK]

Common Mistakes:

• Using setTimeout for repeated calls
• Using infinite while loop causing freeze
• Assuming repeat is a valid function