ES6 Partial Object Destructuring Default Parameters In A Separated Object

Question

Okay so there's a lot of similar questions here but none that actually address this. What if I want to save the default parameters of a function on a separated object, in a separated file and then I want the function to use this default parameters only if not specified when calling it?

Example:

export const defaultParameters = { a: "foo", b: "bar"}

And then in another file:

import {defaultParameters} from ./utils.js

myFunction = function(defaultParameters = {}){
   const {a, b} = defaultParameters;
   console.log(a,b);
}

What I want for it to do is that calling

myFunction({a: "hello"})

should print

"hello", "bar"

But that's not how that works and I'm trying to figure this out. Anyone knows how to achieve this?

Can you use const { a,b } = { ...defaultParameters, ...options }. Since options comes last in that it will override any other parameters. You just made a merged object that you can query. — somethinghere
– somethinghere, Commented Jun 1, 2022 at 14:18
By naming your function's parameter defaultParameters, you're overwriting the imported one. — pistou
– pistou, Commented Jun 1, 2022 at 14:19
You probably can't find anything because what you ask for is impossible. Moreover, an XY problem. Seems like the real thing you're after is have "default fallback values for a parameter object" which is both possible and would give you relevant results. — VLAZ
– VLAZ, Commented Jun 1, 2022 at 14:20

Keith · Accepted Answer · 2022-06-01 14:18:14Z

4

You won't be able to do it directly in the parameter, but you can inside the function..

eg..

import {defaultParameters} from ./utils.js

myFunction = function(params = {}){
   const {a, b} = {...defaultParameters, ...params};
   console.log(a,b);
}

answered Jun 1, 2022 at 14:18

Keith

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7 Comments

Ilijanovic Over a year ago

Well, its actually ugly. Some value depends on some outer value. Usually you try to make a function as pure as possible

Daniel Guedes Over a year ago

Thanks @Keith! I got it to work. @bill.gates why do you think its ugly? What do you mean by a "pure" function? Do you have any alternative way you would approach this problem?

Ilijanovic Over a year ago

@DanielGuedes pure means the function return values are identical for identical arguments. This function has side-effects. If defaultParameters are changed or missing you receive different results by the same input. Sometimes you cannot achieve to make a function pure, but you should try getting towards purity. alternatives would be to pass defaultParameters as an second argument like (params = {}, defaultParameters = {}) and then you pass it when you call it.

Keith Over a year ago

@bill.gates defaultParameters, is obviously a global constant, as such this function is indeed pure, there would be no side effects.

Daniel Guedes Over a year ago

Oh I see, but in my application that's actually my goal. If something is not specified I want the function to grab it from defautParemeters, and as @Keith mentioned it is going to be a global constant and I'm not going to change it dynamically, but I still want the option to manually change it from time to time as my code unravel. Thank you! Never got as quick and efficient reponse as for this question!

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