Array.from doesn't work as a direct callback function in Array#map

Question

I've discovered some very odd behavior from Array.from. It appears that it does not work as a callback function directly when mapped over an array of Array-like objects. I've tested in Chrome.

Here's some test code (ES6):

const fails = () => {
  const x = {
    0: 'help',
    length: 1
  };

  const y = [x].map(Array.from); // will throw an Error
  return y;
};

const works = () => {
  const x = {
    0: 'help',
    length: 1
  };

  const y = [x].map(item => Array.from(item)); // will work
  return y;
};

console.log(works());
console.log(fails());

https://jsfiddle.net/dox6wnya/

This is very peculiar behavior. I'm wondering why this happens.

deceze · Accepted Answer · 2016-04-25 07:25:17Z

4

.map passes three arguments to its callback (currentValue, index, array), and .from accepts three arguments (arrayLike, mapFn, thisArg). The types of arguments don't match and/or produce unexpected results; in particular "0 is not a function", where 0 is the index argument passed to mapFn. The only real compatible argument is the first one, which is why it's the only one you should pass.

edited Apr 25, 2016 at 7:25

answered Apr 25, 2016 at 7:21

deceze♦

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3 Comments

Jamiec Over a year ago

Why do I only see 2 args on Array.map?

deceze Over a year ago

map takes two args but passes three args to callback.

PitaJ Over a year ago

Ah. That makes sense. Thanks.

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