reduce function giving "Uncaught TypeError: Cannot set property 'title' of undefined"

Question

I'm trying to reshape a JSON object in javascript using the reduce function.

As i understand, each time a new property is added to an object in javascript, it must be declared like ac['new_object'] = {};. I'm doing this, so i don't understand why I'm seeing the error: Uncaught TypeError: Cannot set property 'title' of undefined

Reproducible example:

let field_list = [
    {
        "key": "id",
        "val": {
            "boost": 4
        }
    },
    {
        "key": "title",
        "val": {
            "boost": 4
        }
    }
]

field_list.reduce(function(ac, cu){
    ac[cu.key] = {}
    ac[cu.key] = cu.val
}, {})

How can i correctly reduce this JSON object? What was my error here?

Target format:

{
    tales: {
        foo: {"id": 2},
        bar: {"title": 1},
        ...
    }
}

Answer 1

Don't forget to return the accumulator object at the end of the function:

field_list.reduce(function(ac, cu){
  ac[cu.key] = {}
  ac[cu.key] = cu.val
  return ac; // <---- added return statement
}, {})

Without that, undefined will be implicitly returned, and so the next iteration of the loop will have ac equal to undefined