This challenge is to flatten an array without defining any new functions:
// challenge
const arr = [1, [2]]
const flattened = arr.reduce(/*your code here, no function expressions or declarations allowed*/, [])
console.log(flattened) // expected: [1, 2]
My solution doesn't work, but what really bugs me is that I have no idea where the 0 is coming from:
// Solution 1
const arr = [1, [2]]
const flattened = arr.reduce(Function.prototype.call.bind(Array.prototype.concat), [])
console.log(flattened) // unexpected result: [1, 0, 1, [1], 2, 1, 1, [1]]
I'd expected the code to behave like the follow, which works as expected:
// Solution 2
const arr = [1, [2]]
const flattenReducer = (x, y) => Function.prototype.call.bind(Array.prototype.concat)(x, y)
const flattened = arr.reduce(flattenReducer, [])
console.log(flattened) // logs the expected result: [1, 2]
I tested in Node, Chrome, and Firefox and got the same results.
So where is the 0 coming from and why do Solution 1 and Solution 2 produce different values for flattened?
reduce's callback doesn't only have two arguments. In fact it has four: accumulator, current value, index, and the original array.
Your solution #1 is equivalent to
arr.reduce(function (x, y, z, a) {
return Function.prototype.call.bind(Array.prototype.concat)(x, y, z, a);
}, []);
or equivalently
arr.reduce(function (x, y, z, a) {
return x.concat(y, z, a);
}, []);
which is wrong.
Also note that both of your solutions doesn't actually flatten arrays with more than two dimensions.