readable.filter(fn[, options])

• fn <Function> | <AsyncFunction> 从流中过滤块的函数。
  • data <any> 来自流的数据块。
  • options <Object>
    • signal <AbortSignal> 如果流被销毁则中止，允许提前中止 fn 调用。
• options <Object>
  • concurrency <number> 一次调用流的最大并发调用 fn。 默认值: 1。
  • signal <AbortSignal> 如果信号被中止，则允许销毁流。
• 返回: <Readable> 使用谓词 fn 过滤的流。

此方法允许过滤流。 对于流中的每个块，将调用 fn 函数，如果它返回真值，则该块将被传给结果流。 如果 fn 函数返回 promise，则该 Promise 将被 await。

import { Readable } from 'node:stream';
import { Resolver } from 'node:dns/promises';

// 使用同步谓词。
for await (const chunk of Readable.from([1, 2, 3, 4]).filter((x) => x > 2)) {
  console.log(chunk); // 3, 4
}
// 使用异步谓词，单次最多进行 2 个查询。
const resolver = new Resolver();
const dnsResults = Readable.from([
  'nodejs.org',
  'openjsf.org',
  'www.linuxfoundation.org',
]).filter(async (domain) => {
  const { address } = await resolver.resolve4(domain, { ttl: true });
  return address.ttl > 60;
}, { concurrency: 2 });
for await (const result of dnsResults) {
  // 在解析的 dns 记录上记录超过 60 秒的域。
  console.log(result);
}

• fn <Function> | <AsyncFunction> a function to filter chunks from the stream.
  • data <any> a chunk of data from the stream.
  • options <Object>
    • signal <AbortSignal> aborted if the stream is destroyed allowing to abort the fn call early.
• options <Object>
  • concurrency <number> the maximum concurrent invocation of fn to call on the stream at once. Default: 1.
  • signal <AbortSignal> allows destroying the stream if the signal is aborted.
• Returns: <Readable> a stream filtered with the predicate fn.

This method allows filtering the stream. For each chunk in the stream the fn function will be called and if it returns a truthy value, the chunk will be passed to the result stream. If the fn function returns a promise - that promise will be awaited.

import { Readable } from 'node:stream';
import { Resolver } from 'node:dns/promises';

// With a synchronous predicate.
for await (const chunk of Readable.from([1, 2, 3, 4]).filter((x) => x > 2)) {
  console.log(chunk); // 3, 4
}
// With an asynchronous predicate, making at most 2 queries at a time.
const resolver = new Resolver();
const dnsResults = Readable.from([
  'nodejs.org',
  'openjsf.org',
  'www.linuxfoundation.org',
]).filter(async (domain) => {
  const { address } = await resolver.resolve4(domain, { ttl: true });
  return address.ttl > 60;
}, { concurrency: 2 });
for await (const result of dnsResults) {
  // Logs domains with more than 60 seconds on the resolved dns record.
  console.log(result);
}