Why does Ramda work in this example where native Array.map or Lodash do not? - ramda.js

Question

In the process of writing an example for this SO question, this question came up:
Why does the native Array.map throw an error when used like this:
[tmp1, tmp2].map(fs.createReadStream)
.forEach(stream => stream.pipe(jsonStream));
fs.js:1664
throw new TypeError('"options" argument must be a string or an object');
^
TypeError: "options" argument must be a string or an object
at new ReadStream (fs.js:1664:11)
at fs.createReadStream (fs.js:1649:10)
at Array.map (native)
Similarly with lodash… but it works fine with ramda.
// Same error:
_.map([tmp1, tmp2], fs.createReadStream)
.forEach(stream => stream.pipe(jsonStream));
// Works fine:
R.map(fs.createReadStream, [tmp1, tmp2])
.forEach(stream => stream.pipe(jsonStream));
Note, this is the full code from referenced question:
var fs = require('fs');
var path = require('path');
var JSONStream = require('JSONStream');
var tmp1 = path.join(__dirname, 'data', 'tmp1.json');
var tmp2 = path.join(__dirname, 'data', 'tmp2.json');
var jsonStream = JSONStream.parse();
jsonStream.on('data', function (data) {
console.log('---\nFrom which file does this data come from?');
console.log(data);
});
[tmp1, tmp2].map(p => {
return fs.createReadStream(p);
}).forEach(stream => {
stream.pipe(jsonStream);
});
The second argument of fs.createReadStream should be undefined no?

Answer 1

This is most likely due to Array.prototype.map and _.map passing three arguments to the provided mapping function (the value, the index and the collection), while R.map only passes the value.
In your example, fs.createReadStream is being given the array index as its second argument where it expects an options object or string instead, causing the "options" argument must be a string or an object error. If you want to use Array.prototype.map or _.map in this way, you'll need to wrap the method call in a function to prevent the extra arguments:
[tmp1, tmp2].map(p => fs.createReadStream(p))