Watch for changes and run a callback


Runes provide a handy way of running a callback when reactive values change: $effect. It automatically detects when inner values change, and re-runs the callback.

$effect is great, but sometimes you want to manually specify which values should trigger the callback. Svelte provides an untrack function, allowing you to specify that a dependency shouldn't be tracked, but it doesn't provide a way to say that only certain values should be tracked.

watch does exactly that. It accepts a getter function, which returns the dependencies of the effect callback.



Runs a callback whenever one of the sources change.

	import { watch } from "runed";
let count = $state(0);
watch(() => count, () => {

The callback receives two arguments: The current value of the sources, and the previous value.

	let count = $state(0);
watch(() => count, (curr, prev) => {
    console.log(`count is ${curr}, was ${prev}`);

You can also send in an array of sources:

	let age = $state(20);
let name = $state("bob");
watch([() => age, () => name], ([age, name], [prevAge, prevName]) => {
  // ...

watch also accepts an options object.

	watch(sources, callback, {
  // First run will only happen after sources change when set to true.
  // By default, its false.
  lazy: true


watch.pre is similar to watch, but it uses $effect.pre under the hood.


In case you want to run the callback only once, you can use watchOnce and watchOnce.pre. It functions identically to the watch and watch.pre otherwise, but it does not accept any options object.


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