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

function hasContext(key: any): boolean

Parameters

key · any · required

The key to check for in parent component contexts.

Returns

hasContext · boolean

Returns true if a parent component has set a context with this key, false otherwise.

Example

Conditional Context Usage

<!-- 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

<!-- 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

<!-- ParentWithoutContext.svelte -->
<script>
  import Child from './Child.svelte';
</script>

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

Optional Context Pattern

<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

<!-- 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