Returns a provided default pipe if the initial value is falsy.

default.pipe.ts

import { Pipe, PipeTransform } from '@angular/core';

@Pipe({name: 'default', pure: true})
export class DefaultPipe implements PipeTransform {
  transform(value: any, defaultValue: any): any {
    return value || defaultValue;
  }
}

example.html

<img [src]="img.src | default:'https://source.unsplash.com/random/640x480'">