Reading lines in JavaScript

readline turns a stream into lines, without loading the file.

import { createReadStream } from "node:fs";
import { mkdtemp, writeFile } from "node:fs/promises";
import { tmpdir } from "node:os";
import { join } from "node:path";
import { createInterface } from "node:readline";

const dir = await mkdtemp(join(tmpdir(), "lines-"));
const path = join(dir, "log.txt");
await writeFile(path, "INFO start\nWARN slow\nINFO done\n");

const lines = createInterface({
  input: createReadStream(path),
  crlfDelay: Infinity,
});

const counts = new Map();
for await (const line of lines) {
  const level = line.split(" ")[0];
  counts.set(level, (counts.get(level) ?? 0) + 1);
}
console.log([...counts.entries()].sort());

How it works

1. crlfDelay: Infinity treats CRLF as one break.
2. The interface is async iterable.
3. This is how you process a log larger than memory.

Keywords and builtins used here

• Map
• await
• const
• for
• from
• import
• of

The run, in numbers

Lines

21

Characters to type

643

Tokens

160

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 91 seconds.

Type this snippet

Step 2 of 2 in Streams & lines, step 6 of 18 in Node.js.

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Reading lines in other languages

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