Javascript function and expression function

Question

This tiny Javascript program returns the sha256 hash of its text.

const shasum = require('crypto').createHash('sha256');
const stream = require('fs').createReadStream(__filename);

stream.on('error', function() { console.log('Error.'); });
stream.on('data', function (chunk) { shasum.update(chunk); }); /* <--- data line */
stream.on('end', function() {
    const sha = shasum.digest('base64');
    console.log(`The sha is ${sha}`);
});

Executed with Nodejs Erbium, it works as expected.

However, after writing it I thought that the function expression was not needed and so I changed the data line with the following:

stream.on('data', shasum.update);

And it crashes with a horrific error message:

  if (state[kFinalized])
           ^

TypeError: Cannot read property 'Symbol(kFinalized)' of undefined
    at ReadStream.update (internal/crypto/hash.js:78:12)
    at ReadStream.emit (events.js:311:20)
    at addChunk (_stream_readable.js:294:12)
    at readableAddChunk (_stream_readable.js:275:11)
    at ReadStream.Readable.push (_stream_readable.js:209:10)
    at internal/fs/streams.js:210:12
    at FSReqCallback.wrapper [as oncomplete] (fs.js:487:5)

Javascript is very flexible with function calls, but according to the documentation the stream.on data call should pass only one parameter.

Why is the behavior different?

Does this answer your question? [How to access the correct \`this\` inside a callback?](https://stackoverflow.com/questions/20279484/how-to-access-the-correct-this-inside-a-callback) — VLAZ, Aug 06 '20 at 06:36
Basically if you just want to use a function reference, you need `shasum.update.bind(shasum)`. However, I personally don't like it - it looks odd to have a "sandwich" of the context. If you want it shorter, just use an arrow function `chunk => shasum.update(chunk)` — VLAZ, Aug 06 '20 at 06:38

score 1 · Accepted Answer · answered Aug 06 '20 at 06:33

The issue is the context.

The stream will bind the data function to the stream itself

stream.on('data', function (chunk) {
  console.log(this) // it is the `stream`
  shasum.update(chunk)
})

In this case, the shasum.update is bind to the stream, so the update function will not work:

function update(data, encoding) {
  const state = this[kState];
  if (state[kFinalized])
    throw new ERR_CRYPTO_HASH_FINALIZED();

To let it works you must write this statement:

stream.on('data', shasum.update.bind(shasum))

Javascript function and expression function

1 Answers1