Array.from TypeError: 0 is not a function

Question

I'm getting a strange error when trying to pass Array.from to Array.prototype.map.

let fn = Array.from.bind(Array); // [Function: bound from]

fn('test') // [ 't', 'e', 's', 't' ]

['test'].map(s => fn(s)) // [ [ 't', 'e', 's', 't' ] ]

['test'].map(fn) // TypeError: 0 is not a function

Full Error:

TypeError: 0 is not a function
    at Function.from (native)
    at Array.map (native)
    at repl:1:10
    at REPLServer.defaultEval (repl.js:260:27)
    at bound (domain.js:287:14)
    at REPLServer.runBound [as eval] (domain.js:300:12)
    at REPLServer.<anonymous> (repl.js:429:12)
    at emitOne (events.js:95:20)
    at REPLServer.emit (events.js:182:7)
    at REPLServer.Interface._onLine (readline.js:211:10)

What's going on?

Answer 1

map calls its callback with three arguments: The entry, the index, and the object being iterated over. Array.from expects that if a second argument is given to it, it's a mapping function, and so tries to call it on each "element" it's building the array out of. The index, 0 on the first call, isn't a function, so Array.from fails.

Put it another way, the equivalent of

['test'].map(fn)

is not

['test'].map(e => fn(e))

but rather

['test'].map((e, i, a) => fn(e, i, a))

...where e is the entry, i is its index, and a is the "array" being traversed by map. Since i is not a function, Array.from fails.

You get the same sort of thing with several other array functions, such as forEach, some, ...

---

If you do this a lot, you may find it useful to have a function you can use to filter all but the first argument:

function passOneArg(f) {
    return function(a) { return f.call(this, a); };
}

which you can use like this:

['test'].map(passOneArg(fn))

Or possibly even

function limitArgs(f, count) {
    return function() {
        return f.apply(this, Array.prototype.slice.call(arguments, 0, count));
    };
}

then

['test'].map(limitArgs(fn, 1))

Those are, of course, two of the worst function names on the planet, but you get the idea... :-)