kfmc_website/src/app/shared/number.directive.ts

import { Directive, ElementRef, HostListener } from '@angular/core';

@Directive({
  selector: '[appNumber]'
})
export class NumberDirective {

  // Allow decimal numbers and negative values
  private regex: RegExp = new RegExp(/^-?[0-9]+(\.[0-9]*){0,1}$/g);
  // Allow key codes for special events. Reflect :
  // Backspace, tab, end, home
  private specialKeys: Array<string> = [ 'Backspace', 'Tab', 'End', 'Home', '-' ];
 
 constructor(private el: ElementRef) {
  }
  @HostListener('keydown', [ '$event' ])

  onKeyDown(event: KeyboardEvent) {
    // Allow Backspace, tab, end, and home keys
    if (this.specialKeys.indexOf(event.key) !== -1) {
    return;
    }

    if(this.el.nativeElement.value == '-') {
      alert('ffs');
    }

    let current: string = this.el.nativeElement.value;
    let next: string = current.concat(event.key);
    if (next && !String(next).match(this.regex)) {
    event.preventDefault();
    }
  }
}