typescript defining array with named objects

Question

I am struggling to define a type for the array with named objects

below is the sample code.

now i have to define the type for the s in typescript

 let s =[12,3,4]
 s['data']={'x':0}
 console.log(s) // [12,3,4,data:{'x':0}]

now how to define the type for the s

can anyone please guide

While technically possible, an array doesn't have "named objects"! — deceze
– deceze ♦, Commented Sep 3, 2020 at 13:40
Typescript and Javascript. An array is an object, like almost everything, and you can set arbitrary properties on objects, which is why this technically works. But data is not in any way something to do with an array. — deceze
– deceze ♦, Commented Sep 3, 2020 at 13:44
please refer to observablehq.com/@d3/stacked-area-chart in that there is an array series please expand it.. you will see that array has object named with data — Shiva Sai
– Shiva Sai, Commented Sep 3, 2020 at 13:46
ShivaSai - Right. @deceze said (twice) that it's possible. It's just not a good idea in most cases. — T.J. Crowder
– T.J. Crowder, Commented Sep 3, 2020 at 13:48

T.J. Crowder · Accepted Answer · 2020-09-03 13:43:51Z

2

It's probably best not to do this. But you can do it, by using an intersection type, in this case:

type AugmentedNumberArray = number[] & {
    data?: {x: number};
};

That's a type that's both a number array and an object with an optional data property that can be an object with an x property with a number value.

Playground Link

But as soon as you do something like s2 = s.map(v => v * 2), you'll lose the data property, because map (like all array methods) only deals with the array-like part of the object, not any other properties it may have.

Instead, though, I'd recommend defining an object that has both the named properties (data or whatever) and an array of numbers:

interface MyType {
    data?: {x: number};
    values: number[];
}

Then using it like this:

let s: MyType = {values: [12,3,4]};
s['data'] = {'x':0}; // Or idiomatically: s.data = ...
console.log(s);

Playground Link

answered Sep 3, 2020 at 13:43

T.J. Crowder

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2 Comments

Shiva Sai Over a year ago

hey can i define the data to the array in declaration it self? instead of reassigning after wards

T.J. Crowder Over a year ago

@ShivaSai - I'm afraid not, there's no way to create a non-element property with an array literal. The best you can do is something like this: s = Object.assign([12, 3, 4], {data: {x: 0}}); More: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

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