How do I use a function created in a parameter in Javascript?

Question

I'm trying to pass this deepMap([4,5, [3,4,[2]]], x => x + 5) into this function:

const deepMap = (arr, fn) => {
  return arr.reduce((first, second) => first.concat(Array.isArray(second) ? [deepMap(second)] : fn), []);  
}

previously it worked like this:

const deepMap = (arr, fn) => {
  return arr.reduce((first, second) => first.concat(Array.isArray(second) ? [deepMap(second)] : second + 5), []);  
}

but that didn't allow me to use the function in the second parameter. I know it needs to work like the second example, but the only way I can think to change it is like it is in the first example. I have tried numerous variations experimentally but I keep getting an error or the wrong answer.

Inside the deepMap function the function passed as an argument is named fn, so you call it like fn(xyz). If you want subsequent recursive calls to deepMap to also be able to call it, you have to pass it on: deepMap(second, fn). — Jordan Running
– Jordan Running, Commented Jun 1, 2017 at 20:00
adding fn to the call deepMap definitely helped. Thank You so much!!!! — Lauren Grinder
– Lauren Grinder, Commented Jun 1, 2017 at 20:04

Oleg Imanilov · Accepted Answer · 2017-06-01 20:23:53Z

1

You should put return value of function instead of function itself. Like this:

const deepMap = (arr, fn) => {
  return arr.reduce((first, second) => first.concat(Array.isArray(second) ? [deepMap(second, fn)] : fn(second)), []);  
}

edited Jun 1, 2017 at 20:23

answered Jun 1, 2017 at 20:03

Oleg Imanilov

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2 Comments

Egor Stambakio Over a year ago

Your code doesn't work. Inner deepMap should be called like deepMap(second, fn), not deepMap(second)

Oleg Imanilov Over a year ago

Ok, fixed, Thanks wostex

Egor Stambakio · Accepted Answer · 2017-06-01 20:04:49Z

0

Just pass both array and function to recursive deepMap calls and invoke fn(cur) instead of just pointing to it.

const deepMap = (arr, fn) => {
  return arr.reduce((first, second) => first.concat(Array.isArray(second) ? [deepMap(second, fn)] : fn(second)), []);  
}

const res = deepMap([4,5, [3,4,[2]]], x => x + 5);

console.log(res)

edited Jun 1, 2017 at 20:04

answered Jun 1, 2017 at 20:02

Egor Stambakio

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2 Comments

mhodges Over a year ago

No explanation is not helpful

Egor Stambakio Over a year ago

@mhodges I've added it while you were writing this comment.

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