last

签名: last(predicate: function): Observable

根据提供的表达式，在源 observable 完成时发出它的最后一个值。

与 last 对应的操作符是 first！

示例

示例 1: 序列中的最后一个值

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import { from } from 'rxjs/observable/from';
import { last } 'rxjs/operators';
const source = from([1, 2, 3, 4, 5]);
// 没有参数则发出最后一个值
const example = source.pipe(last());
// 输出: "Last value: 5"
const subscribe = example.subscribe(val => console.log(`Last value: ${val}`));

示例 2: 最后一个通过 predicate 函数的值

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import { from } from 'rxjs/observable/from';
import { last } 'rxjs/operators';
const source = from([1, 2, 3, 4, 5]);
// 发出最后一个偶数
const exampleTwo = source.pipe(last(num => num % 2 === 0));
// 输出: "Last to pass test: 4"
const subscribeTwo = exampleTwo.subscribe(val =>
  console.log(`Last to pass test: ${val}`)
);

示例 3: 使用结果选择器的 last

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import { from } from 'rxjs/observable/from';
import { last } 'rxjs/operators';
const source = from([1, 2, 3, 4, 5]);
// 提供第二个参数，它是一个可选的投射函数
const exampleTwo = source.pipe(
  last(
    v => v > 4,
    v => `The highest emitted number was ${v}`
  )
);
// 输出: 'The highest emitted number was 5'
const subscribeTwo = exampleTwo.subscribe(val => console.log(val));

示例 4: 使用默认值的 last

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jsFiddle )

import { from } from 'rxjs/observable/from';
import { last } 'rxjs/operators';
const source = from([1, 2, 3, 4, 5]);
// 没有值通过的话则发出默认值
const exampleTwo = source.pipe(last(v => v > 5, v => v, 'Nothing!'));
// 输出: 'Nothing!'
const subscribeTwo = exampleTwo.subscribe(val => console.log(val));

其他资源

• last - 官方文档
• 过滤操作符: takeLast, last - André Staltz

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