Passing a prototype method to a Promise's then method

Question

I have a promisified method I called readFilePromise, that resolves to a Buffer object from fs' readFile method. When I execute the following line

return readFilePromise(filePath).then(Buffer.prototype.toString.call);

I get the following error:

TypeError: undefined is not a function

However, when I execute the block:

return readFilePromise(filePath).then((data) => {
    return Buffer.prototype.toString.call(data);
});

I get no error and the code executes fine.

In my mind they should be the same. Am I missing something obvious?

node v6.10.1

Do you really want to use `call` instead of doing `data.toString()`? — Bergi, Mar 31 '17 at 22:11
No, i wanted to use the first example. the second was for demonstration purposes only. — Ray Wadkins, Apr 03 '17 at 13:50

score 1 · Accepted Answer · answered Mar 31 '17 at 21:40

Buffer.prototype.toString.call is just Function.prototype.call which calls this using first object as a context. In your first example this inside call call will be undefined.

You need to bind call to Buffer.prototype.toString like this Buffer.prototype.toString.call.bind(Buffer.prototype.toString).

return readFilePromise(filePath)
   .then(Buffer.prototype.toString.call.bind(Buffer.prototype.toString))

Passing a prototype method to a Promise's then method

1 Answers1