change

Change detection operator, only triggers subsequent operations when the result of the differ function differs from the previous one.

Type Definition

change: <T>(differ: (value: T | undefined) => any) =>
  (observable$: Observable<T>) =>
    Observable<T>

Parameters

• differ: Function to extract the value for comparison from the source data
  • Parameter: value: T | undefined - The current data in the stream
  • Return value: any - The value used for comparison

Details

• The first emission always triggers (since there is no previous value to compare)
• Only triggers when the result of differ is different from the previous one
• The emitted value is the original complete data, not the part returned by differ
• Uses strict equality (===) to detect changes in the result of the differ function

Examples

import { $ } from 'fluth'

const promise$ = $<{ user: { name: string; age: number } }>()
promise$.pipe(change((value) => value?.user.name)).then(() => console.log('Username changed'))

promise$.next({ user: { name: 'Alice', age: 25 } }) // Output: Username changed
promise$.next({ user: { name: 'Bob', age: 25 } }) // Output: Username changed
promise$.next({ user: { name: 'Bob', age: 30 } }) // No output
promise$.next({ user: { name: 'Alice', age: 30 } }) // Output: Username changed

import { $ } from 'fluth'

const promise$ = $<{ items: number[] }>()
promise$
  .pipe(change((value) => value?.items?.length))
  .then(() => console.log('Array length changed'))

promise$.next({ items: [1, 2] }) // Output: Array length changed
promise$.next({ items: [1, 2, 3] }) // Output: Array length changed
promise$.next({ items: [1, 2, 3] }) // No output
promise$.next({ items: [1] }) // Output: Array length changed