What is let-* in Angular 2 templates?

update Angular 5

ngOutletContext was renamed to ngTemplateOutletContext

See also CHANGELOG.md @ angular/angular

original

Templates (<template>, or <ng-template> since 4.x) are added as embedded views and get passed a context.

With let-col the context property $implicit is made available as col within the template for bindings.With let-foo="bar" the context property bar is made available as foo.

For example if you add a template

<ng-template #myTemplate let-col let-foo="bar">  <div>{{col}}</div>  <div>{{foo}}</div></ng-template><!-- render above template with a custom context --><ng-template [ngTemplateOutlet]="myTemplate"             [ngTemplateOutletContext]="{                                           $implicit: 'some col value',                                           bar: 'some bar value'                                        }"></ng-template>

See also this answer and ViewContainerRef#createEmbeddedView.

*ngFor also works this way. The canonical syntax makes this more obvious

<ng-template ngFor let-item [ngForOf]="items" let-i="index" let-odd="odd">  <div>{{item}}</div></ng-template>

where NgFor adds the template as an embedded view to the DOM for each item of items and adds a few values (item, index, odd) to the context.

See also Using $implict to pass multiple parameters

The Angular microsyntax lets you configure a directive in a compact, friendly string. The microsyntax parser translates that string into attributes on the <ng-template>. The let keyword declares a template input variable that you reference within the template.