Creating a new object from an existing one, or from an array, can have multiple solutions in JavaScript. I ran into a scenario where I had an object where the values are an array. I wanted to take this and create a new object (so I could store state related to the keys), so I wanted to dump the values and replace them for something else.

Start:

const obj = { "a" : [1,2,3], "b": [1,2,3] }

Destination:

const newObj = { "a" : "val", "b": "val" }

The values could be whatever, the point is to take the keys and then a different set of values.

I can do this with brute force by enumerating over the keys in the object and assigning them to a new one:

let newObj = {}
for (key in obj) {
    newObj[key] = "val"
}

There’s a newer (ES2019) utility, Object.fromEntries, which takes an iterable (arrays, Maps, but not an Object that you haven’t implemented an iterator on…) that can be used here:

Object.fromEntries(Object.keys(obj).map(x => [x, "foo"]))

To break it down:

// create an object from the provided entries. cool cool.
Object.fromEntries(Object.keys(obj).map(x => [x, "val"]))

// give me the keys of the obj Object; ["a", "b"]
Object.fromEntries(Object.keys(obj).map(x => [x, "val"]))

// take those keys and make tuples; [ ["a", "val" ], ["b", "val" ] ]
Object.fromEntries(Object.keys(obj).map(x => [x, "val"]))

To get to our result of

{ a: "val", b: "val" }

Writing it out, I’m hoping to understand this more so I can understand it quickly when I see it. But I found it neat to stumble on a new-to-me utility.

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