Derived values with $: in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $: syntax do in Svelte?

It creates a reactive statement that automatically re-runs whenever the variables it depends on change.

beginner

How do you declare a derived value that updates when its dependencies change in Svelte?

Use the $: label followed by an assignment or expression that depends on reactive variables.

beginner

Given <code>let count = 1;</code> and <code>$: double = count * 2;</code>, what happens when <code>count</code> changes?

The variable double automatically updates to twice the new count value.

intermediate

Can you use $: to run code that does not assign a value but reacts to changes?

Yes, you can write reactive statements without assignments to run code whenever dependencies change, like logging or calling functions.

intermediate

Why is using $: for derived values better than manually updating variables in Svelte?

Because it keeps your code simple and automatic, avoiding bugs from forgetting to update values and improving readability.

What triggers a reactive statement with $: to re-run in Svelte?

A When any variable it depends on changes

B Only when the component mounts

C When the user clicks a button

D When the page reloads

Reactive statements re-run automatically whenever any of their dependent variables change.

Which of these is a correct way to declare a derived value in Svelte?

A $: total = price * quantity;

B let $total = price + quantity;

C const $: sum = price + quantity;

D function $: sum() { return price + quantity; }

The $: label is used before an assignment to create a reactive derived value.

Can you have multiple reactive statements with $: in one Svelte component?

A Only if they are inside functions.

B No, only one $: statement is allowed.

C Yes, you can have many reactive statements.

D Only if they use different variable names.

Svelte allows multiple reactive statements to handle different derived values or side effects.

What happens if a reactive statement does not assign a value but calls a function?

A The function never runs.

B The function runs whenever dependencies change.

C The function runs only once.

D It causes an error.

Reactive statements can run code like functions whenever their dependencies update.

Why should you prefer $: reactive statements over manual updates?

A They only work with numbers.

B They run slower but are easier to write.

C They require more code but are clearer.

D They automatically keep values in sync and reduce bugs.

Reactive statements automatically update derived values, making code simpler and less error-prone.

Explain how the $: syntax helps manage derived values in Svelte.

Think about how changes in one variable affect others without manual code.

Describe a scenario where you would use a reactive statement without assigning a value.

Consider when you want to run code on changes but don't need a new variable.