Angular2屏蔽SSN

Question

我已经使用Angular2指令屏蔽了电话号码和SSN。但我需要用星号（）替换屏蔽的SSN。 例如：输入SSN：123-45-6789，输出SSN： - - ****。 我拥有：输入SSN：123456789，输出SSN：123-45-6789。 工作的Plunker代码：https://plnkr.co/edit/XrqvSuge65Oi7IovBa8Y?p=preview

masking.component.ts

import {Component, Directive, Output, EventEmitter} from '@angular/core';
import {FormBuilder, FORM_DIRECTIVES, NgControl} from '@angular/common';
import {PhoneMask} from './phoneMask.directive';
import {SsnMask} from './ssnMask.directive';


@Component({
  selector: 'masking',
  providers: [],
  template: `
  <form [ngFormModel]="form">
    Phone:<input type="text" phone [(ngModel)]="data" (rawChange)="rawPhone=$event" ngControl="phone" maxlength="14">
    SSN:<input type="text" ssn [(ngModel)]="ssndata" (rawChange)="rawSsn=$event" ngControl="ssn" maxlength="11">  
  </form>
  <div>Raw Numbers: {{rawPhone}}</div>
  <div>SSN: {{rawSsn}}</div>
  `, 
  directives: [PhoneMask,SsnMask]
})
export class Masking {
  form :any;
  constructor(private fb:FormBuilder) {
    this.form=fb.group({
      phone:[''],
      ssn:['']
    })
  }
}

ssnMask.directive.ts

import {Directive,Output,EventEmitter} from '@angular/core';
import {NgControl} from '@angular/common';
@Directive({
    selector:'[ngModel][ssn]',
    host:{
        '(ngModelChange)':'onInputChange($event)',
        '(keydown.backspace)':'onInputChange($event.target.value,true)'
    }
})

export class SsnMask{
    constructor(public model: NgControl){}

    @Output() rawChange:EventEmitter<string> = new EventEmitter<string>();
    onInputChange(event,backspace){
        var newVal=event.replace(/\D/g,'');
        var rawValue= newVal;

        if(backspace){
            newVal=newVal.substring(0, newVal.length);
        }

        if(newVal.length==0){
            newVal='';
        }
        else if(newVal.length<=3){
            newVal=newVal.replace(/^(\d{0,3})/, '$1');
        }
        else if(newVal.length <=6){
            newVal=newVal.replace(/^(\d{0,3})(\d{0,2})/, '$1-$2');
        }
        else if(newVal.length<=9){
            newVal=newVal.replace(/^(\d{0,3})(\d{0,2})(\d{0,4})/, '$1-$2-$3')
        }
        this.model.valueAccessor.writeValue(newVal);
        this.rawChange.emit(rawValue);

    }
}