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

i've discovered odd behavior array.from. appears not work callback function directly when mapped on array of array-like objects. i've tested in chrome.

here's test code (es6):

const fails = () => {   const x = {     0: 'help',     length: 1   };    const y = [x].map(array.from); // throw error   return y; };  const works = () => {   const x = {     0: 'help',     length: 1   };    const y = [x].map(item => array.from(item)); // work   return y; };  console.log(works()); console.log(fails()); 

https://jsfiddle.net/dox6wnya/

this peculiar behavior. i'm wondering why happens.

.map passes three arguments callback (currentvalue, index, array), , .from accepts 3 arguments (arraylike, mapfn, thisarg). types of arguments don't match and/or produce unexpected results; in particular "0 not function", 0 index argument passed mapfn. real compatible argument first one, why it's 1 should pass.