node/test/parallel/test-stream-preprocess.js

'use strict';
const common = require('../common');
const assert = require('assert');

const fs = require('fs');
const rl = require('readline');
const fixtures = require('../common/fixtures');

const BOM = '\uFEFF';

// Get the data using a non-stream way to compare with the streamed data.
const modelData = fixtures.readSync('file-to-read-without-bom.txt', 'utf8');
const modelDataFirstCharacter = modelData[0];

// Detect the number of forthcoming 'line' events for mustCall() 'expected' arg.
const lineCount = modelData.match(/\n/g).length;

// Ensure both without-bom and with-bom test files are textwise equal.
assert.strictEqual(fixtures.readSync('file-to-read-with-bom.txt', 'utf8'),
                   `${BOM}${modelData}`
);

// An unjustified BOM stripping with a non-BOM character unshifted to a stream.
const inputWithoutBOM =
  fs.createReadStream(fixtures.path('file-to-read-without-bom.txt'), 'utf8');

inputWithoutBOM.once('readable', common.mustCall(() => {
  const maybeBOM = inputWithoutBOM.read(1);
  assert.strictEqual(maybeBOM, modelDataFirstCharacter);
  assert.notStrictEqual(maybeBOM, BOM);

  inputWithoutBOM.unshift(maybeBOM);

  let streamedData = '';
  rl.createInterface({
    input: inputWithoutBOM,
  }).on('line', common.mustCall((line) => {
    streamedData += `${line}\n`;
  }, lineCount)).on('close', common.mustCall(() => {
    assert.strictEqual(streamedData, modelData);
  }));
}));

// A justified BOM stripping.
const inputWithBOM =
  fs.createReadStream(fixtures.path('file-to-read-with-bom.txt'), 'utf8');

inputWithBOM.once('readable', common.mustCall(() => {
  const maybeBOM = inputWithBOM.read(1);
  assert.strictEqual(maybeBOM, BOM);

  let streamedData = '';
  rl.createInterface({
    input: inputWithBOM,
  }).on('line', common.mustCall((line) => {
    streamedData += `${line}\n`;
  }, lineCount)).on('close', common.mustCall(() => {
    assert.strictEqual(streamedData, modelData);
  }));
}));