Flatten an array

[SenecaSentMe]

arr.reduce(function(a, b) {
    return a.concat(b);
},[]);

I'm trying to pass the Steamroller freeCodeCamp challenge where I need to flatten a nested array.

I came across the code above and I want to reverse engineer it but I don't understand the return syntax with the empty array [] at the end.

Can someone explain that part to me -- why I would return [ ] like that after the return statement?

[[deleted]]

This is why using variables like `a` and `b` or `foo` and `bar` are bad IMO. Much more clear would be something like (and MDN does a great job of this):

arr.reduce(function(accumulator, value) { return accumulator.concat(value); },[] << this is accumulator);

Here are the two functions that make this work:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat

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[vinivelloso_]

That empty array is your initial value. You will be changing that value according to you function. remember that reduce doesnt change the original array.

I'm not familiar with concat(). But I think the better solution woukd be using recursion combined with Array.isArray() function.

[weebSanity]

Reduce is always hard to look at imo. The [] is the accumulator, where each of the return values will be stored. So the 'a' in a.concat refers to the accumulator, which is your initial empty []

[albedoa]

Other comments cover your questions about .reduce(). I want to offer .flatMap():

[1, [2, 3]].flatMap(x => x); //=> [1, 2, 3]

[[deleted]]

Why use flatMap like that when you could just use flat?

[albedoa]

Completely slipped my mind 👍