view child @angular 5+ token two arguments ('local reference name',static: false|true)

@ViewChild('nameInput', { static: false }) nameInputRef: ElementRef;

to know the difference between true and false check this

static - whether or not to resolve query results before change detection runs (i.e. return static results only). If this option is not provided, the compiler will fall back to its default behavior, which is to use query results to determine the timing of query resolution. If any query results are inside a nested view (e.g. *ngIf), the query will be resolved after change detection runs. Otherwise, it will be resolved before change detection runs.

In ng8 , you can manually set when to access the child component in the parent component. When you set static to true, it means the parent component only get the definition of the component in the onInit hook: Eg:

 // You got a childComponent which has a ngIf/for tag
ngOnInit(){
  console.log(this.childComponent);
}

ngAfterViewInit(){
  console.log(this.childComponent);
}

If static is false , then you only get the defination in the ngAfterViewInit(), in ngOnInit(), you'll get undefined.

So as a rule of thumb you can go for the following:

• { static: true } needs to be set when you want to access the ViewChild in ngOnInit.

• { static: false } can only be accessed in ngAfterViewInit. This is also what you want to go for when you have a structural directive (i.e. *ngIf) on your element in your template.

From the angular docs

static - whether or not to resolve query results before change detection runs (i.e. return static results only). If this option is not provided, the compiler will fall back to its default behavior, which is to use query results to determine the timing of query resolution. If any query results are inside a nested view (e.g. *ngIf), the query will be resolved after change detection runs. Otherwise, it will be resolved before change detection runs.

It may be a better idea to use static:true if the child does not depend on any conditions. If the visibility of element changes, then static:false may give better results.

PS: Since its a new feature, we may need to run benchmarks for performance.

Edit

As mentioned by @Massimiliano Sartoretto , github commit may give you more insights.

Came here because a ViewChild was null in ngOnInit after upgrading to Angular 8.

Static queries are filled before ngOnInit, while dynamic queries (static: false) are filled after. In other words, if a viewchild is now null in ngOnInit after you set static: false, you should consider changing to static: true or move code to ngAfterViewInit.

See https://github.com/angular/angular/blob/master/packages/core/src/view/view.ts#L332-L336

The other answers are correct and explain why this is the case: Queries dependant on structural directives, e.g. a ViewChild reference inside an ngIf, should run after the conditional of this directive has been resolved, ie after change detection. However, one may safely use static: true and thus resolve the queries before ngOnInit for unnested references. Imho this particular case bears mentioning as a null exception could likely be the first way you'll encounter this particularity, as it was for me.

View child

... Can use it for template element reference.

...for outside of particular component reference.

With decorator style syntax.. @ViewChild( selector) reference : ElementRef || QueryList.

Of specific component or element reference.

Use it in AfterViewInIt().

We could use it in Oninit().

But this for specific to use ngAfterViewInit().

Lastly {static : false} should be placed in @ViewChild( Useme , { static : false})... for template variable reference.

The variable in template file looks like. #Useme.