Q:

ngClass is not working in angular 4

ngClass directive generating the following error
ERROR Error: Uncaught (in promise): Error: Template parse errors:
Can't bind to 'ngClass' since it isn't a known property of 'li'.

code snippet.
 <li [ngClass]="{'active':true}"><a href="">Home</a></li>

NB: I already imported the common module
import {CommonModule} from "@angular/common"

Library version angular 4.0.1

A:

As @Nedim suggested, if you are using [ngClass] within a feature module, you'd need to make sure that CommonModule is being added to the imports: [] array property of @NgModule({}):
import {CommonModule} from "@angular/common"

@NgModule({
    imports: [CommonModule]
})
export class SomeModule {}

To conditionally add a single class based on a boolean value/expression I'd recommend you use the syntax [class.someClass]="condition". Here is the documentation for Class binding.
<li [class.active]="true"><a href="">Home</a></li>

For example, to evaluate against a boolean class property to conditionally apply "active" CSS class:
TS:
export class App {
  foo: boolean = true;
}

HTML:
<div [class.active]="foo">Foobar</div>

Here is a plunker demonstrating the functionality in action.
Note: If you are trying to apply some type of active styling related to the router, you could use the RouterLinkActive binding. The following example would apply the CSS class "active" when the user has activated the path "/heroes".
<a routerLink="/heroes" routerLinkActive="active">Heroes</a>

