Today I Learned

TIL, 2022-04-02, CD Musings, memo-decorator


  • closest method traverses the Element and its parents until it finds a node that matches the provided selector. If no element matches, the closest() method returns null.
  • How to use trackBy with ngFor Reference
    • On each ngDoCheck triggered for the ngForOf, Angular checks what objects have changed. It uses differs for this process and each differ uses the trackBy function to compare the current object with the new one.
  • Prevent angular component from re-rendering unless data has changed Reference
  • Looking at it from the other side. Every setTimeout can actually rerender tons of components (the ones with the default change detection strategy). If you know in advance that it doesn’t need to rerender any components, you could in fact block that as well:
onInit() {
  let counter = 0;
  setInterval( () => {
    // this is something an OnPush strategy won't detect.
  }, 1000);
  • Change detection doesn't fire in setTimeout with ChangeDetectionStrategy.OnPush Reference
    • Really good question.
    • The bound click event will mark the component and its ancestors as dirty.
    • After onClick(), Angular will call ApplicationRef.tick() to do CD. setTimeout hasn’t executed yet.
    • After CD completes, components are no longer dirty, then setTimeout executes.
    • The CD cycle works before setTimeout works.
  • HTMLElement.focus() - this works by itself but Angular has focusMonitor.
  • Getting the activeElement Reference
    • document.activeElement.
    • Can create a directive that has HostListener on focus.
  selector: '[componentFocus]'
export class ComponentFocus {
  componentFocus() {
    console.log(`component has focus, element:`, this.el);

  constructor(private el: ElementRef) {
  • Or use on-focus.
  • Checking if two vars have the same reference? Reference was added in ES2015. Or do the triple equals.

How do I manually trigger change detection in Angular?


  • True “manual” change detection would be OnPush + detectChanges, detach, and markForCheck() in the components.
  • When OnPush change detection is enabled, Angular will not detect changes by analyzing each template expression. It will only detect changes to @Input values as a whole, not the properties of an object.
  • Using OnPush, Angular will only check “when notified”. Scenarios:
    • @Input reference has changed. Angular compares the object reference, not individual properties. This is why immutable data is the way to go. Every time you change your data, you’re making a copy with a new object reference.
    • Component or its children raise an event.
    • A bound observable in the template changes.
    • Manual via (detectChanges()).
      • detectChanges immediately runs change detection.
      • markForCheck() temporarily adds that component and its children to the default Angular change detection for one cycle.

memo decorator with Angular Pipe

Reference Reference

  • memo-decorator enhances the caching of pipe’s transform. Makes the pipe share results for all methods of that pipe.
  • Reference - the method is bound to the prototype, not to the instance. This fails for some but works for some.



const debounce = (callback, wait) => {
  let timeoutId = null;
  return (...args) => {
    timeoutId = window.setTimeout(() => {
      callback.apply(null, args);
    }, wait);
  • Lodash would help, but this means you have 1 more utility library, plus you’re reducing reliance on third-party code.
  • React: useMemo.
  • Pass in a callback function.
    • Cancel any pre-existing timeout.
    • Schedule a new timeout, based on the amount of time indicated by the wait argument.

How to Inject Document in Angular


Accessibility Made Easy with Angular CDK


This project is maintained by daryllxd