Add values as array to object key-pair

Question

I want to add values to empty object to their correct key that is received dynamically.

Obviously the following does not work because I replace the existing value of that key with the latest value:

let obj = {};

let key;
let value;

key = // from selectors 
value = // from selectors 

obj[key] = value;

How can I "push" the new value to that key instead of replacing it?

For example if value1 and value2 were chosen after 2 iterations with key1 then the end result of the object would be

{"key1": ['value1', 'value2']}

Just putting it out there - this question has already been answered. You should mark one of them as a solution before all of stackoverflow posts their own solution. — no_modules
– no_modules, Commented Dec 28, 2022 at 12:00

Nick Parsons · Accepted Answer · 2022-12-28 12:03:13Z

3

There are a few ways you can achieve this. A common way is to set obj[key] equal to itself, defaulting it to an empty array with ?? [] (nullish coalescing) if it is null/undefined, ie: nullish), and then using .concat() to return a new array with the added value:

obj[key] = (obj[key] ?? []).concat(value); // ?? can be replaced with `||` for better browser support

Or use ??= (nullish coalescing assignment) to set obj[key] to an empty array if it's nullish, and then push into that array .push() to update the existing array

(obj[key] ??= []).push(value);

The above works because obj[key] ?? = [] will set obj[key] to [] if obj[key] is undefined/null, and then return the reference to the array stored at obj[key], that being the array we just created or the array that already exists at obj[key]. We can then .push() into this array reference which updates it in obj

edited Dec 28, 2022 at 12:03

answered Dec 28, 2022 at 11:54

Nick Parsons

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no_modules · Accepted Answer · 2022-12-28 11:50:13Z

1

Try this:

if (obj[key] === undefined) obj[key] = []
obj[key].push(value)

answered Dec 28, 2022 at 11:50

no_modules

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Samathingamajig · Accepted Answer · 2022-12-28 11:59:02Z

1

When you're assigning, just add to the array or create one if it doesn't exist

obj[key] = [...(obj[key] ?? []), value];

,

obj[key] = (obj[key] ?? []).concat(value);

, or

if (!obj[key]) obj[key] = [];
obj[key].push(value);

edited Dec 28, 2022 at 11:59

answered Dec 28, 2022 at 11:51

Samathingamajig

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