How to create a Javascript object from an array of known keys, but with the values initialized to null?

Question

I have a JS object from which I will extract an array of keys (using Object.keys()) and then I want to use those same keys to create a different object, but this time with its values initialized to null.

In Python, I would do something like:

list_of_keys = ["key_1", "key_2"]

# The * unpacks the list, could be thought as a JS spread operator
new_dict = dict.fromkeys(*list_of_keys)

And I would retrieve a new dictionary with all its values initialized to None as a default.

I don't seem to find a similar property or method in Javascript, however.

EDIT: If using ESLint, @CerebralFart answer (the for... of...) might trigger a complaint from the linter. It can be addressed here.

CerebralFart · Accepted Answer · 2022-03-03 22:06:12Z

1

There's no need to make this more complicated than a simple for loop:

const object = { key_1: 'value_1', key_2: 'value_2', key_3: 'value_3' };
const newObject = {};

for (const key in object) {
  newObject[key] = null
}

console.log(newObject);

Or, if you only have the keys

const keys = ['key_1', 'key_2', 'key_3'];
const newObject = {};

for (const key of keys) {
  newObject[key] = null
}

console.log(newObject);

ETA: You could also use Object.fromEntries with some mapping. Depending on what you want to do with the object, this may be marginally faster.

const object = { key_1: 'value_1', key_2: 'value_2', key_3: 'value_3' };

const newObject = Object.fromEntries(Object.keys(object).map(key => [key, null]));

console.log(newObject);

edited Mar 3, 2022 at 22:06

answered Mar 3, 2022 at 21:56

CerebralFart

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1 Comment

LeperAffinity666 Over a year ago

I ended up doing exactly that, even although I really think that the reduce approach is quite ingenious. But I tend to favor readability and performance, so I think I'll stick to the ole for loop. Edited a word, confused ingenious with ingenuous

NeNaD · Accepted Answer · 2022-03-03 21:49:50Z

1

You can use reduce method to do it:

const object = { key_1: 'value_1', key_2: 'value_2', key_3: 'value_3' };

const new_object = Object.keys(object).reduce((accumulator, currentValue)=> {
  accumulator[currentValue] = null;
  return accumulator;
}, {});

console.log(new_object)

answered Mar 3, 2022 at 21:49

NeNaD

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2 Comments

CerebralFart Over a year ago

That's some magnificent abuse of the reduce function. A simple for-in loop would be much more appropriate

LeperAffinity666 Over a year ago

This answer also works, but (at least for me) it's difficult to understand at first sight. I guess I'll stick to the for loop. But this code is quite ingenious, nice trick

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