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The <svelte:options> element allows you to specify compiler options.

We'll use the immutable option as an example. In this app, the <Todo> component flashes whenever it receives new data. Clicking on one of the items toggles its done state by creating an updated todos array. This causes the other <Todo> items to flash, even though they don't end up making any changes to the DOM.

We can optimise this by telling the <Todo> component to expect immutable data. This means that we're promising never to mutate the todo prop, but will instead create new todo objects whenever things change.

Add this to the top of the Todo.svelte file:

<svelte:options immutable={true} />

You can shorten this to <svelte:options immutable/> if you prefer.

Now, when you toggle todos by clicking on them, only the updated component flashes.

The options that can be set here are:

  • immutable={true} — you never use mutable data, so the compiler can do simple referential equality checks to determine if values have changed
  • immutable={false} — the default. Svelte will be more conservative about whether or not mutable objects have changed
  • accessors={true} — adds getters and setters for the component's props
  • accessors={false} — the default
  • namespace="..." — the namespace where this component will be used, most commonly "svg"
  • customElement="..." — the name to use when compiling this component as a custom element

Consult the API reference for more information on these options.