first

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。

与 first 对应的操作符是 last！

示例

(
example tests
)

示例 1: 序列中的第一个值

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import { from } from 'rxjs/observable/from';
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

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import { from } from 'rxjs/observable/from';
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

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const source = Rx.Observable.from([1, 2, 3, 4, 5]);
// 使用可选的 projection 函数
const example = source.first(
  num => num % 2 === 0,
  (result, index) => `First even: ${result} is at index: ${index}`
);
// 输出: "First even: 2 at index: 1"
const subscribe = example.subscribe(val => console.log(val));

示例 4: 利用默认值

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

其他资源

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

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