array filter 泛型返回具体元素类型

[wangkailang]

array filter 泛型返回具体元素类型

is 指定推导结果类型：

filter<S> S 可以忽略不填，但是返回值 value is S 必须手写

type Alice = {
  name: string;
  department: string;
}
type Bob = {
  name: string;
  department: boolean;
}
type Carl = {
  name: string;
  department: number;
}

type AllName = Alice | Bob | Carl;

const employees: AllName[] = [
  { name: 'Alice', department: 'accounting' },
  { name: 'Bob', department: true },
  { name: 'Carl', department: 10 },
];

const isAlice = (obj: AllName): obj is Alice => obj.name === 'Alice';

const result = employees.filter(isAlice);
// result => Alice[];