pipe 错误：类型“数字”不可分配给类型“字符串”

Question

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

@Pipe({
  name: 'concat',
  pure: true,
})
export class ConcatPipe implements PipeTransform {
  public transform(value: string, args: string[]): string {
    return value?.concat(...args);
  }
}

When I try to concating in ngFor:

 <mat-option *ngFor="let name of names$ | async; index as i" [value]="i" [id]="'firstName' | concat: [i]">

I have this pipe where I concat things in the template side, but i get this error:

Type 'number' is not assignable to type 'string'

How can I fix or extend my pipe?

Answer 1

It means you receive numbers, not strings. You should first try to log what happens in value and args . Then you would have to cast numbers into string:

let strValue: string;
if (typeof value === 'number') {
  strValue = parseInt(value);
} else {
  strValue = value;
}

same goes for the args array:

argsStrs: string[] = args.map(arg => {
  if (typeof arg === 'number') {
    return parseInt(arg);
  } else {
    return arg;
  }
}