JavaScript runtime overview - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print 'Hello, world!' to the console.

Javascript

console.[1]('Hello, world!');

Drag options to blanks, or click blank then click option'

Alog

Bprint

Cwrite

Decho

Attempts:

3 left

💡 Hint

Common Mistakes

Using console.print or console.write which do not exist in JavaScript.

Trying to use echo which is from other languages.

The console.log method prints messages to the console in JavaScript.

2fill in blank

medium

Complete the code to create a new Promise that resolves with 'Done'.

Javascript

const promise = new Promise((resolve, reject) => { resolve([1]); });

Drag options to blanks, or click blank then click option'

AError

Bfalse

Cnull

D'Done'

Attempts:

3 left

💡 Hint

Common Mistakes

Passing Error to resolve instead of reject.

Using null or false which are not the intended resolved value.

The Promise resolves with the string 'Done' using resolve('Done').

3fill in blank

hard

Fix the error in the async function to await the promise correctly.

Javascript

async function fetchData() { const result = [1] promise; return result; }

Drag options to blanks, or click blank then click option'

Apromise.then()

Bresolve

Cawait

Dasync

Attempts:

3 left

💡 Hint

Common Mistakes

Using promise.then() without await inside async function.

Trying to use resolve or async keywords incorrectly.

The await keyword pauses the async function until the promise resolves.

4fill in blank

hard

Fill both blanks to create a setTimeout that logs 'Hi' after 1 second.

Javascript

setTimeout(() => { console.[1]('Hi'); }, [2]);

Drag options to blanks, or click blank then click option'

Alog

B1000

C500

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using console.error instead of console.log.

Using 500 instead of 1000 for delay.

console.log prints the message, and 1000 means 1000 milliseconds (1 second) delay.

5fill in blank

hard

Fill all three blanks to create a Promise that rejects with 'Fail' after 2 seconds.

Javascript

const failPromise = new Promise(([1], [2]) => { setTimeout(() => { [2]('[3]'); }, 2000); });

Drag options to blanks, or click blank then click option'

Aresolve

Breject

CFail

DSuccess

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping resolve and reject parameters.

Calling resolve instead of reject to indicate failure.

Using 'Success' instead of 'Fail' as rejection message.

The Promise executor takes resolve and reject. We call reject('Fail') after 2 seconds to reject the promise.

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