Debugging with {@debug} in Svelte - Performance & Optimization

Performance: Debugging with {@debug}

LOW IMPACT

This affects development speed and debugging efficiency but has minimal impact on page load or rendering performance in production.

Inspecting reactive variables during development

Svelte

{@debug variable}

Automatically logs reactive variable values to the console during development, updating on changes without manual logging.

📈 Performance GainNo runtime overhead in production; improves debugging speed and accuracy

Inspecting reactive variables during development

Svelte

console.log(variable);

Console logging can clutter output and is not integrated with Svelte's reactivity, causing manual updates and potential missed changes.

📉 Performance CostMinimal runtime cost but can slow developer workflow and cause noisy logs

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Using console.log for debugging	0	0	0	[!] OK
Using {@debug} in development	0 (console only)	0	0	[OK] Good for dev
Leaving {@debug} in production	0 (no effect)	0	0	[OK] Ignored

Rendering Pipeline

The {@debug} tag logs debug info to the console only during development. It logs reactively with variable changes but does nothing in production builds.

⚠️ BottleneckNone

Optimization Tips

1Use {@debug} only during development to inspect reactive variables.

2{@debug} is automatically ignored in production; remove for code cleanliness.

3Prefer {@debug} over console.log for reactive debugging in Svelte.

Performance Quiz - 3 Questions

What is the main performance impact of using {@debug} in Svelte during development?

A Increases JavaScript bundle size significantly

B Triggers console logs on variable changes

C Blocks rendering and delays page load

D Causes layout shifts on every frame

DevTools: Console panel

How to check: Open Console panel to see {@debug} logs during development; Elements panel shows no extra nodes. Verify no logs in production build.

What to look for: Reactive debug logs in Console during dev; absence in production; no DOM impact

Practice

1. What is the main purpose of the {@debug} tag in Svelte?

easy

A. To create a new reactive variable

B. To style variables with CSS inside the component

C. To print variable values to the browser console during app execution

D. To pause the app execution for debugging

Solution

1. Step 1: Understand the role of {@debug}

   The {@debug} tag is used to output variable values to the browser console while the app runs.

2. Step 2: Compare with other options

   Options B, C, and D describe unrelated features: styling, creating variables, or pausing execution, which {@debug} does not do.

3. Final Answer:

   To print variable values to the browser console during app execution -> Option C

4. Quick Check:

   {@debug} shows variables in console = A [OK]

Hint: Remember {@debug} logs variables to console automatically [OK]

Common Mistakes:

• Thinking {@debug} styles variables
• Confusing {@debug} with reactive declarations
• Assuming {@debug} pauses app execution

2. Which of the following is the correct syntax to debug variables count and name using {@debug} in Svelte?

easy

A. {@debug (count, name)}

B. {@debug count; name}

C. {@debug {count, name}}

D. {@debug count, name}

Solution

1. Step 1: Recall {@debug} syntax

   The correct syntax is {@debug} followed by variables separated by commas without parentheses or braces.

2. Step 2: Check each option

   {@debug count, name} matches the correct syntax. Options A, C, and D use invalid parentheses, braces, or semicolons which cause syntax errors.

3. Final Answer:

   {@debug count, name} -> Option D

4. Quick Check:

   Use commas without extra symbols in {@debug} [OK]

Hint: Use commas only, no parentheses or braces in {@debug} [OK]

Common Mistakes:

• Adding parentheses around variables
• Using braces instead of commas
• Separating variables with semicolons

3. Given this Svelte code snippet:

  <script>
    let count = 5;
    let name = 'Svelte';
  </script>
  
  {@debug count, name}

What will you see in the browser console when this component runs?

medium

A. The text 'count, name' printed in the console

B. An object showing { count: 5, name: 'Svelte' }

C. No output because {@debug} is only for development mode

D. A syntax error preventing the app from running

Solution

1. Step 1: Understand {@debug} output

   {@debug} prints the current values of the listed variables as an object in the console.

2. Step 2: Analyze given variables

   Variables count=5 and name='Svelte' will be shown as { count: 5, name: 'Svelte' } in the console.

3. Final Answer:

   An object showing { count: 5, name: 'Svelte' } -> Option B

4. Quick Check:

   {@debug} logs variable values as objects [OK]

Hint: Expect variable values as an object in console [OK]

Common Mistakes:

• Expecting text output instead of object
• Thinking {@debug} disables output in production
• Confusing syntax error with correct usage

4. You wrote this code:

<script>
  let count = 10;
</script>

{@debug count;}

But you get a syntax error. What is the problem?

medium

A. Using a semicolon after the variable inside {@debug} is invalid syntax

B. You must declare count as a reactive variable with $: before debugging

C. The {@debug} tag must be inside a <div> element

D. You forgot to import {@debug} from Svelte

Solution

1. Step 1: Check {@debug} syntax rules

   {@debug} expects variables separated by commas without semicolons or other punctuation.

2. Step 2: Identify the error

   The semicolon after count causes a syntax error because it is not allowed inside {@debug}.

3. Final Answer:

   Using a semicolon after the variable inside {@debug} is invalid syntax -> Option A

4. Quick Check:

   No semicolons inside {@debug} [OK]

Hint: Never put semicolons inside {@debug} brackets [OK]

Common Mistakes:

• Adding semicolons after variables in {@debug}
• Thinking {@debug} needs imports
• Placing {@debug} only inside HTML elements

5. You have a Svelte component with variables user (an object) and items (an array). You want to debug both and see their current values in the console whenever they change. Which is the best way to use {@debug} for this?

hard

A. {@debug user, items} placed inside the component markup

B. {@debug {user, items}} inside a reactive statement

C. {@debug user; items} inside the script tag

D. Use console.log(user, items) instead of {@debug}

Solution

1. Step 1: Recall correct {@debug} usage

   {@debug} is used inside the markup with variables separated by commas, no braces or semicolons.

2. Step 2: Evaluate options

   {@debug user, items} placed inside the component markup: {@debug user, items} placed inside the component markup is correct. Use console.log(user, items) instead of {@debug} uses console.log instead of {@debug}. {@debug {user, items}} inside a reactive statement uses invalid braces and incorrect placement in a reactive statement. {@debug user; items} inside the script tag uses semicolons and wrong placement inside the script tag.

3. Final Answer:

   {@debug user, items} placed inside the component markup -> Option A

4. Quick Check:

   {@debug with comma-separated vars in markup = D [OK]

Hint: Put {@debug} with comma-separated vars inside markup [OK]

Common Mistakes:

• Using braces or semicolons inside {@debug}
• Placing {@debug} inside script tag
• Confusing console.log with {@debug} usage