Question

first

函数签名: first(predicate: function, select: function)

发出第一个值或第一个通过给定表达式的值。

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与 first 对应的操作符是 last！

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示例

( example tests )

示例 1: 序列中的第一个值

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { from } from 'rxjs';
import { first } from 'rxjs/operators';

const source = from([1, 2, 3, 4, 5]);
// 没有参数则发出第一个值
const example = source.pipe(first());
// 输出: "First value: 1"
const subscribe = example.subscribe(val => console.log(`First value: ${val}`));

示例 2: 第一个通过 predicate 函数的值

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { from } from 'rxjs';
import { first } from 'rxjs/operators';

const source = from([1, 2, 3, 4, 5]);
// 发出通过测试的第一项
const example = source.pipe(first(num => num === 5));
// 输出: "First to pass test: 5"
const subscribe = example.subscribe(val =>
  console.log(`First to pass test: ${val}`)
);

示例 3: 使用可选的 projection 函数

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { from } from 'rxjs';
import { first } from 'rxjs/operators';

const source = from([1, 2, 3, 4, 5]);
// 没有值通过的话则发出默认值
const example = source.pipe(first(val => val > 5, 'Nothing'));
// 输出: 'Nothing'
const subscribe = example.subscribe(val => console.log(val));

其他资源

• first :newspaper: - 官方文档
• 过滤操作符: take, first, skip :video_camera: :dollar: - André Staltz

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源码: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/first.ts