<div class="header">
    <a routerLink="" class="brand">QA</a>
    <div class="header_link">
        <a routerLink="question/ask" class="header_link--item" *ngIf="isUser()">Ask a Question</a>
        <a routerLink="signin" class="header_link--item" *ngIf="!isUser()">Sign In</a>
        <a routerLink="signup" class="header_link--item" *ngIf="!isUser()">Sign Up</a>
        <img src="https://unsplash.it/100/100" alt="" class="header_link--btn" *ngIf="isUser()">
        <div class="header_link--dropdown">
            <a href="">Questions</a>
            <a href="">Answers</a>
            <a href="">Log Out</a>
        </div>
    </div>
</div>

import { Directive, Input, OnInit, ElementRef, Renderer2 } from "@angular/core";


@Directive({
    selector : '[menuDropdown]'
})

export class MenuDropdownDirective implements OnInit{

    constructor(private elRef:ElementRef,private renderer:Renderer2){}

    ngOnInit(){

    }

}