Today I Learned

TIL, 2019-12-30

  • What’s the difference between markForCheck() and detectChanges()? Reference
    • There is a case where any thing inside your model has changed but it hasn’t been reflected in the view, so need to notify Angular to detect those changes.
    • It could be because the change detector is detached from the view.
    • It could be because an update has happened but it hasn’t been inside the Angular Zone, and therefore Angular doesn’t know about it. If a third party function has updated your model and you want to update the view after that, you have to do something like this:

   // Let's detect the changes that above function made to the model which Angular is not aware of.;
  • To make this function run inside the Angular change cycle, you can:
    • Wrap it in the Angular zone (
    • Wrap the function inside a setTimeout: setTimeout(this.someFunctionThatIsRunByAThirdPartyCode)
    • If you update the model after the change detection cycle is finished, you get this: “Expression has changed after it was checked”. This means that the model changed and then I ran my change detection to update the view, but there was another function in your code which updated the model again, and I don’t want to run CD again because there is no dirty checking like AngularJS anymore.
  • Proper way to fix: Make sure that update is inside the change detection cycle, do not update the model after that and move your code.
  • Lazy way: Just make Angular run detectChanges.
  • Put the code inside a setTimeout, because setTimeout is patched by zone.

  • markForCheck: Run the change detection if any of these has happened:
    • One of the @input of the component has been completely replaced with a new value, or if the reference of the @Input property has changed together.
    • If you don’t update the object reference, the change detection is not going to run, and the view will not reflect the update/mutation.
    • An event has been fired and the child components emitted an event.

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