offline docs

offline-docs/alpinejs/magics/watch.md

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+# Alpine.js Documentation - Magics
+
+Magics are special properties that are available inside Alpine expressions. They provide convenient access to common functionality.
+
+## [$watch](/magics/watch)
+
+You can "watch" a component property using the `$watch` magic method. For example:
+
+```
+<div x-data="{ open: false }" x-init="$watch('open', value => console.log(value))">
+
+
+n<button @click="open = ! open">Toggle Open</button>
+
+
+n</div>
+```
+
+In the above example, when the button is pressed and `open` is changed, the provided callback will fire and `console.log` the new value:
+
+You can watch deeply nested properties using "dot" notation
+
+```
+<div x-data="{ foo: { bar: 'baz' }}" x-init="$watch('foo.bar', value => console.log(value))">
+
+
+n<button @click="foo.bar = 'bob'">Toggle Open</button>
+
+
+n</div>
+```
+
+When the `<button>` is pressed, `foo.bar` will be set to "bob", and "bob" will be logged to the console.
+
+### [Getting the "old" value](#getting-the-old-value)
+
+`$watch` keeps track of the previous value of the property being watched, You can access it using the optional second argument to the callback like so:
+
+```
+<div x-data="{ open: false }" x-init="$watch('open', (value, oldValue) => console.log(value, oldValue))">
+
+
+n<button @click="open = ! open">Toggle Open</button>
+
+
+n</div>
+```
+
+### [Deep watching](#deep-watching)
+
+`$watch` automatically watches from changes at any level but you should keep in mind that, when a change is detected, the watcher will return the value of the observed property, not the value of the subproperty that has changed.
+
+```
+<div x-data="{ foo: { bar: 'baz' }}" x-init="$watch('foo', (value, oldValue) => console.log(value, oldValue))">
+
+
+n<button @click="foo.bar = 'bob'">Update</button>
+
+
+n</div>
+```
+
+When the `<button>` is pressed, `foo.bar` will be set to "bob", and "{bar: 'bob'} {bar: 'baz'}" will be logged to the console (new and old value).
+
+> ⚠️ Changing a property of a "watched" object as a side effect of the `$watch` callback will generate an infinite loop and eventually error.
+
+```
+<!-- 🚫 Infinite loop -->
+
+<div x-data="{ foo: { bar: 'baz', bob: 'lob' }}" x-init="$watch('foo', value => foo.bob = foo.bar)">
+
+
+n<button @click="foo.bar = 'bob'">Update</button>
+
+
+n</div>
+```
+
+Code highlighting provided by [Torchlight](https://torchlight.dev/)