 the component is updated as a result of a state change. When the component receives new prop values or has its local state modified, the component's 

 lifecycle method is called before any updates to the DOM are made. Once the 

 lifecycle method finishes executing, the DOM will be updated with these data changes, which will allow the subsequent call to 

 to have access to a completely synced DOM. Being able to schedule a callback to run at this phase of a component's lifecycle ensures that the value of certain DOM properties, such as a container element's 

, can be cached prior to being updated. Then, these cached values can be used to revert those properties back to what they were originally.

 lifecycle method is used inside of a component:

 is called before the component's initial 

Suppose you are formatting text within an editable element, such as an 

 via a set of controls in a floating tooltip. When you create a selection by dragging your cursor over a subset of the text and click on any one of the tooltip options to transform the text, the selected text should be modified with the selection and focus on the editable element still intact. 

By default, clicking anywhere outside of an editable element will cause it to lose its focus and current text selection. This does not happen when clicking on any of the buttons in the tooltip because of additional code that preserves the selection and focus. 

Visit this simple Svelte REPL demo to see how both the focus and text selection can be immediately restored using the 

 Lifecycle Method - Restore Input Cursor Position

In this demo, there are four buttons, each of which transform the input field's text selection differently:

When any one of these buttons is clicked while there is a text selection in the input field, the 

 function is called. This function transforms the text selection and sets 

 lifecycle method will be called, which checks if 

 is called before the component is mounted) and caches the start and end positions of the text selection. After the DOM is updated and the new 

 is rendered within the input field, the 

 lifecycle method will be called, which will recreate the text selection based on the cached positions and place the focus back on the input field. 

If you have built React applications, then you will have probably noticed how 

. Try rewriting some of the those components that use 

If you want to learn more about Svelte, then check out 

AUTHOR

Ken Chan

In Svelte, a component's beforeUpdate lifecycle method is called before the component is updated as a result of a state change. When the component receives new prop values or has its local state modified, the component's beforeUpdate lifecycle method is called before any updates to the DOM are…

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