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Updated: May 31, 2026

Arrow Function

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Conclusion

Arrow Function — free browser-based JavaScript practice

■ What this exercise teaches Define an arrow function called double that returns its argument doubled. Passing 5 should output 10.

Why it matters in real projects

Arrow functions use the form (argument) => return value. Similar patterns appear daily in code review and production UI work.

Step-by-step workflow

1. Read the guide and understand the goal of "Arrow Function" 2. Review the JavaScript editor pane and start from the starter code 3. Open hints if stuck; compare your diff with the sample solution 4. Click Run and inspect preview/console output 5. Auto-check looks for output containing: 10. 6. After success, continue with adjacent exercises in the same category

Core concept explained

Arrow functions use the form (argument) => return value.

Technical focus

Key JavaScript APIs: console.log.

Hint before you peek at the solution

const double = (x) => x * 2;

Sample solution walkthrough

JavaScript APIs used: console.log. Verify spelling and async/await usage. Explanation: Arrow functions use the form (argument) => return value. Hint recap: const double = (x) => x * 2;

Solution code

📝 Sample code

const double = (x) => x * 2;
console.log(double(5));

Common mistakes for this exact task

- Skipping the hint: const double = (x) => x * 2;

Troubleshooting

Q: Preview does not change after Run A: Ensure you edited the JavaScript pane and clicked Run. Q: Looks correct but check fails A: Auto-check looks for output containing: 10. Look for typos, full-width spaces, or unclosed tags. Q: Console shows errors A: Open DevTools Console and fix bracket/quote mismatches at the reported line. Q: Hard to type on mobile A: Use landscape mode or continue on desktop—the same URL keeps progress in localStorage.

Interview / portfolio tips

- Explain the techniques in "Arrow Function" in your own words within 30 seconds - Practice walking through each line of the sample solution on a whiteboard - Connect this task to a real component you would ship - Mention responsive or accessibility trade-offs - Describe when you use: console.log

What to do after you pass

Move to the next exercise in the js-basic track, then continue the learning roadmap at /en/learning/js-basic/.

Keywords

Free hands-on exercise covering Arrow function, js-basic, elementary, console.log, arrow function—search-friendly guide for Arrow Function.

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■ Exercise task

Define an arrow function called double that returns its argument doubled. Passing 5 should output 10.

Hint if stuck

const double = (x) => x * 2;

15pt

Arrow function

Define an arrow function called double that returns its argument doubled. Passing 5 should output 10.

Code

Preview

Errors & Console

Click error to highlight line. ✕=error ⚠=warning ›=log.

Roadmap · All guides

Deep dive

"Arrow Function" is a elementary-level JavaScript exercise. Define an arrow function called double that returns its argument doubled. Passing 5 should output 10. Run it in-browser with automatic checking.

Arrow functions use the form (argument) => return value. Similar patterns appear daily in code review and production UI work.

Pair this with the js-basic roadmap. Exercise #49 reinforces core js-basic skills.

Step-by-step workflow

Read the guide and understand the goal of "Arrow Function"
Review the JavaScript editor pane and start from the starter code
Open hints if stuck; compare your diff with the sample solution
Click Run and inspect preview/console output
Auto-check looks for output containing: 10.
After success, continue with adjacent exercises in the same category

How to read the sample solution

JavaScript APIs used: console.log. Verify spelling and async/await usage. Explanation: Arrow functions use the form (argument) => return value. Hint recap: const double = (x) => x * 2;

Real-world & career relevance

JS basics precede React, Vue, and Node. Variables, functions, and arrays appear in daily code review.

Common mistakes

Skipping the hint: const double = (x) => x * 2;
Typos in console.log
Loose == between string and number
Reassigning const
Missing semicolons causing odd line breaks

Troubleshooting

Preview does not change after Run: Ensure you edited the JavaScript pane and clicked Run.
Looks correct but check fails: Auto-check looks for output containing: 10. Look for typos, full-width spaces, or unclosed tags.
Console shows errors: Open DevTools Console and fix bracket/quote mismatches at the reported line.
Hard to type on mobile: Use landscape mode or continue on desktop—the same URL keeps progress in localStorage.

Interview & portfolio tips

Explain the techniques in "Arrow Function" in your own words within 30 seconds
Practice walking through each line of the sample solution on a whiteboard
Connect this task to a real component you would ship
Mention responsive or accessibility trade-offs
Describe when you use: console.log

Glossary

const: Constant binding—no reassignment.
function: Reusable block of code.
console.log: Print to the developer console.

Keywords covered on this page

Free hands-on exercise covering Arrow function, js-basic, elementary, console.log, arrow function—search-friendly guide for Arrow Function.

Related dev tools

Learning roadmap

Learning roadmap →

Related exercises

Frequently asked questions

Is "Arrow function" free?

This exercise is free and runs entirely in your browser. No account or credit card required.

Difficulty and time?

"Arrow function" is level "elementary". Read the guide first; beginners can usually finish within 30 minutes. Allow 5–15 minutes per exercise. Use hints, then the solution if stuck.

What after a correct answer?

After a correct answer, use "Next exercise". When you finish a category, continue on the <a href="/en/learning/js-basic/">learning roadmap</a>.

Works on mobile?

Works on mobile browsers, but typing code is easier on a desktop.

Where are other exercises?

All exercises: <a href="/en/learn/">/en/learn/</a> with category filters.

Key takeaway for "Arrow function"?

Arrow functions use the form (argument) => return value. Hint: const double = (x) => x * 2;

Where does JavaScript Basics fit?

JS basics precede React, Vue, and Node. Variables, functions, and arrays appear in daily code review.

Step-by-step for "Arrow Function"?

1. Read the guide and understand the goal of "Arrow Function" 2. Review the JavaScript editor pane and start from the starter code 3. Open hints if stuck; compare your diff with the sample solution 4. Click Run and inspect preview/console output 5. Auto-check looks for output containing: 10. 6. After success, continue with adjacent exercises in the same category

How to read the sample solution?

JavaScript APIs used: console.log. Verify spelling and async/await usage. Explanation: Arrow functions use the form (argument) => return value. Hint recap: const double = (x) => x * 2;

Something is not working?

Preview does not change after Run → Ensure you edited the JavaScript pane and clicked Run. Looks correct but check fails → Auto-check looks for output containing: 10. Look for typos, full-width spaces, or unclosed tags. Console shows errors → Open DevTools Console and fix bracket/quote mismatches at the reported line. Hard to type on mobile → Use landscape mode or continue on desktop—the same URL keeps progress in localStorage.

Career / interview angle?

Explain the techniques in "Arrow Function" in your own words within 30 seconds Practice walking through each line of the sample solution on a whiteboard Connect this task to a real component you would ship Mention responsive or accessibility trade-offs Describe when you use: console.log

Related dev tools?

<a href="/en/tools/json-formatter/">JSON Formatter</a>, <a href="/en/tools/regex/">Regex Tester</a>

Prerequisites?

Read the <a href="/en/learning/js-basic/">Learning roadmap</a> chapter first.

Can I change the starter code?

Yes—if it works. Understanding the starter first, then adding minimal diffs, speeds up learning.

What do the points mean?

15pt is a difficulty hint, not a score you must maximize.