docs/alpinejs/magic-function-$watch.md

$watch
======

You can "watch" a component property using the `$watch` magic method. For example:

    <div x-data="{ open: false }" x-init="$watch('open', value => console.log(value))">    <button @click="open = ! open">Toggle Open</button></div>
    <div x-data="{ open: false }" x-init="$watch('open', value => console.log(value))">
        <button @click="open = ! open">Toggle Open</button>
    </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="$watch('foo.bar', value => console.log(value))">    <button @click="foo.bar = 'bob'">Toggle Open</button></div>
    <div x-data="{ foo: { bar: 'baz' }}" x-init="$watch('foo.bar', value => console.log(value))">
        <button @click="foo.bar = 'bob'">Toggle Open</button>
    </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="$watch('open', (value, oldValue) => console.log(value, oldValue))">    <button @click="open = ! open">Toggle Open</button></div>
    <div x-data="{ open: false }" x-init="$watch('open', (value, oldValue) => console.log(value, oldValue))">
        <button @click="open = ! open">Toggle Open</button>
    </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="$watch('foo', (value, oldValue) => console.log(value, oldValue))">    <button @click="foo.bar = 'bob'">Update</button></div>
    <div x-data="{ foo: { bar: 'baz' }}" x-init="$watch('foo', (value, oldValue) => console.log(value, oldValue))">
        <button @click="foo.bar = 'bob'">Update</button>
    </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="$watch('foo', value => foo.bob = foo.bar)">    <button @click="foo.bar = 'bob'">Update</button></div>
    <!-- 🚫 Infinite loop -->
    <div x-data="{ foo: { bar: 'baz', bob: 'lob' }}" x-init="$watch('foo', value => foo.bob = foo.bar)">
        <button @click="foo.bar = 'bob'">Update</button>
    </div>

[← $store](/magics/store)

[$dispatch →](/magics/dispatch)

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