angular - How do I combine a template reference variable with ngIf?

Question

<div *ngIf="true" myHighlight #tRefVar="myHighlight"></div>
<div>tRefVar is {{tRefVar.foo}}</div>

Even though the *ngIf is true, I get a Cannot read property 'foo' of undefined. If I remove the *ngIf, it works fine!

I tried using the Elvis operator tRefVar?.foo, which resolved the error, but then it never updates with the value.

https://plnkr.co/edit/5rsXygxK1sBbbkYdobjn?p=preview

What am I doing wrong?

question from:https://stackoverflow.com/questions/44811132/how-do-i-combine-a-template-reference-variable-with-ngif

Answer 1

As Tobias Bosch said

A variable declared inside of an *ngIf cannot be used outside of the *ngIf

https://github.com/angular/angular/issues/6179#issuecomment-233374700

Only the opposite way (i.e. declare a variable inside of *ngIf and use it outside of *ngIf) is not working, and won't work by design.

https://github.com/angular/angular/issues/6179#issuecomment-233579605

Why is it so?

1) Without *ngIf

Let's see at this template

<h2 myHighlight #tRefVar="myHighlight">tRefVar is {{tRefVar.foo}}</h2>
<div>tRefVar is {{tRefVar?.foo}}</div>

angular will create the following viewDefinition for that:

function View_App_0(_l) {
  return jit_viewDef1(0,[(_l()(),jit_textDef2(null,['
      '])),(_l()(),jit_elementDef3(0,
      null,null,2,'h2',[['myHighlight','']],null,null,null,null,null)),jit_directiveDef4(16384,
      [['tRefVar',4]],0,jit_HighlightDirective5,[jit_ElementRef6],null,null),(_l()(),
      jit_textDef2(null,['tRefVar is ',''])),(_l()(),jit_textDef2(null,['
      '])),
      (_l()(),jit_elementDef3(0,null,null,1,'div',[],null,null,null,null,null)),(_l()(),
          jit_textDef2(null,['tRefVar is ',''])),(_l()(),jit_textDef2(null,['
  ']))],
      null,function(_ck,_v) {
        var currVal_0 = jit_nodeValue7(_v,2).foo;
        _ck(_v,3,0,currVal_0);
        var currVal_1 = ((jit_nodeValue7(_v,2) == null)? null: jit_nodeValue7(_v,2).foo);
        _ck(_v,6,0,currVal_1);
      });
}

there is no embedded view here. All in one View_App_0. And we can see here our expression {{tRefVar?.foo}}

var currVal_1 = ((jit_nodeValue7(_v,2) == null)? null: jit_nodeValue7(_v,2).foo);

it takes value from node with index 2

jit_directiveDef4(16384,
  [['tRefVar',4]],0,jit_HighlightDirective5,[jit_ElementRef6],null,null),(_l()(),
  jit_textDef2(null,['tRefVar is ','']))

that declared in the same view

2) With *ngIf

Then let's change template as follows

<h2 *ngIf="true" myHighlight #tRefVar="myHighlight">tRefVar is {{tRefVar.foo}}</h2>
<div>tRefVar is {{tRefVar?.foo}}</div>

The output will be the following

function View_App_1(_l) {
  return jit_viewDef1(0,[(_l()(),jit_elementDef2(0,null,null,2,'h2',[['myHighlight',
      '']],null,null,null,null,null)),jit_directiveDef3(16384,[['tRefVar',4]],0,jit_HighlightDirective4,
      [jit_ElementRef5],null,null),(_l()(),jit_textDef6(null,['tRefVar is ','']))],
      null,function(_ck,_v) {
        var currVal_0 = jit_nodeValue7(_v,1).foo;
        _ck(_v,2,0,currVal_0);
      });
}
function View_App_0(_l) {
  return jit_viewDef1(0,[(_l()(),jit_textDef6(null,['
'])),(_l()(),jit_anchorDef8(16777216,
      null,null,1,null,View_App_1)),jit_directiveDef3(16384,null,0,jit_NgIf9,[jit_ViewContainerRef10,
      jit_TemplateRef11],{ngIf:[0,'ngIf']},null),(_l()(),jit_textDef6(null,['
'])),
      (_l()(),jit_elementDef2(0,null,null,1,'div',[],null,null,null,null,null)),(_l()(),
          jit_textDef6(null,['tRefVar is ',''])),(_l()(),jit_textDef6(null,['
  ']))],
      function(_ck,_v) {
        var currVal_0 = true;
        _ck(_v,2,0,currVal_0);
      },function(_ck,_v) {
        var _co = _v.component;
        var currVal_1 = ((_co.tRefVar == null)? null: _co.tRefVar.foo);
        _ck(_v,5,0,currVal_1);
      });
}

Angular created embedded view View_App_1 apart to View_App_0 . And our expression {{tRefVar?.foo}} has turned into

var currVal_1 = ((_co.tRefVar == null)? null: _co.tRefVar.foo);

it just becames component property because there is no node that will reference to this template variable in View_App_0. It's gone to embedded view View_App_1

var currVal_0 = jit_nodeValue7(_v,1).foo;

So we cannot refer to template variable that has been declared in embedded view outside of embedded view.

How to solve it?

1) Use visibility flag like [hidden] or css class instead of *ngIf

2) Move your expression inside embedded view where tRefVar is declared

<ng-container *ngIf="true">
  <h2 myHighlight #tRefVar="myHighlight">tRefVar is {{tRefVar.foo}}</h2>
  <div>tRefVar is {{tRefVar?.foo}}</div>
</ng-container>

3) Use @ViewChild because it will represent component property. Or use @ViewChildren