Angular2 directive, constructor vs onInit

Question

I am simply trying to figure out what the difference between the constructor and the onInit function is in a Directive in Angular2. To me both seems to be used when the object is created, so why would you not only use the constructor?

have a look at this http://stackoverflow.com/a/35763811/5043867 — Pardeep Jain, Mar 12 '16 at 16:00

score 3 · Accepted Answer · answered Feb 28 '16 at 08:49

3

The first one (constructor) is related to the class instantiation and has nothing to do with Angular2. I mean a constructor can be used on any class. You can put in it some initialization processing for the newly created instance.

The second one corresponds to a lifecycle hook of Angular2 components:

ngOnChanges is called when an input or output binding value changes
ngOnInit is called after the first ngOnChanges

So you should use ngOnInit if initialization processing relies on bindings of the component (for example component parameters defined with @Input), otherwise the constructor would be enough...

answered Feb 28 '16 at 08:49

Thierry Templier

198,364
44
396
360

So is there a reason to not always use onInit? – rablentain Feb 28 '16 at 09:51
For example if you want to use injected objects without setting them as class properties... – Thierry Templier Feb 28 '16 at 09:53
Injection is done at the constructor level. But using ngOnInit for other initializations is fine ;-) – Thierry Templier Feb 28 '16 at 10:10

Angular2 directive, constructor vs onInit

1 Answers1