> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/sveltejs/svelte/llms.txt
> Use this file to discover all available pages before exploring further.

# hasContext()

> Check whether a given key has been set in the context of a parent component.

Checks whether a given `key` has been set in the context of a parent component. Must be called during component initialisation.

## Signature

```ts theme={null}
function hasContext(key: any): boolean
```

## Parameters

<ParamField path="key" type="any" required>
  The key to check for in parent component contexts.
</ParamField>

## Returns

<ResponseField name="hasContext" type="boolean">
  Returns `true` if a parent component has set a context with this key, `false` otherwise.
</ResponseField>

## Example

### Conditional Context Usage

```svelte theme={null}
<!-- Child.svelte -->
<script>
  import { hasContext, getContext } from 'svelte';

  let theme;
  
  if (hasContext('theme')) {
    theme = getContext('theme');
  } else {
    // Fallback to default theme
    theme = {
      color: 'gray',
      mode: 'light'
    };
  }
</script>

<div style="color: {theme.color}">
  Current theme: {theme.mode}
</div>
```

### Parent Component With Context

```svelte theme={null}
<!-- ParentWithContext.svelte -->
<script>
  import { setContext } from 'svelte';
  import Child from './Child.svelte';

  setContext('theme', {
    color: 'blue',
    mode: 'dark'
  });
</script>

<Child />
<!-- Will use blue/dark theme -->
```

### Parent Component Without Context

```svelte theme={null}
<!-- ParentWithoutContext.svelte -->
<script>
  import Child from './Child.svelte';
</script>

<Child />
<!-- Will use gray/light fallback theme -->
```

## Optional Context Pattern

```svelte theme={null}
<script>
  import { hasContext, getContext } from 'svelte';

  // Provide optional feature enhancement if context is available
  const analytics = hasContext('analytics') 
    ? getContext('analytics') 
    : null;

  function trackEvent(name) {
    // Only track if analytics context is available
    if (analytics) {
      analytics.track(name);
    }
  }

  function handleClick() {
    trackEvent('button-clicked');
    // ... rest of click handler
  }
</script>

<button onclick={handleClick}>Click me</button>
```

## Library Context Pattern

```svelte theme={null}
<!-- CustomComponent.svelte -->
<script>
  import { hasContext, getContext } from 'svelte';

  // Check if component is used within a library's context provider
  if (!hasContext('my-library-config')) {
    throw new Error(
      'CustomComponent must be used within a LibraryProvider'
    );
  }

  const config = getContext('my-library-config');
</script>
```

## Notes

* Must be called during component initialisation (not in callbacks or effects)
* Use this to provide fallback values when context may not be available
* Useful for creating reusable components that work both with and without context
* Helps avoid errors when accessing context that might not exist