how to check type of variable in typescript +angular?

just use typeof(variable); so in your case:

console.log(typeof(this.a));

Try 'instanceof' or 'is':

a instanceof Abc;

See also: Class type check with typescript

Interfaces are erased at runtime so there will be no trace of the interface in any runtime call. You can either use a class instead of an interface (classes exist at runtime and obey instanceof

class Abc {
    private noLiterals: undefined;
    constructor(public name: string) { }
}
@Component({
    selector: 'my-app',
    templateUrl: './app.component.html',
    styleUrls: ['./app.component.css']
})
export class AppComponent {
    name = 'Angular 6';
    a: Abc = new Abc( "sss")

    constructor() {
        console.log(this.a instanceof Abc) // Will be true 
    }
}

Or you can do structural checking to see if the properties of Abc are present at runtime in the object:

export class AppComponent {
    name = 'Angular 6';
    a: Abc = { name: "sss" }

    constructor() {
        console.log('name' in this.a) // Will be true 
    }
}