How can I get properties of a type as an array in Typescript?

Question

Given the following type:

class Foo {
    constructor(
        private one: string, 
        private two: string, 
        private three: string) {
    }
}

How can I have an array whose values are the type's properties?

e.g. I need to be able to get an array as:

['One', 'Two', 'Three']

Note I need to have the properties extracted from a type and not an instance otherwise I could simply use Object.keys(instanceOfFoo).

where is the array defined? Is each one an array or is there a different array property? How do you want to use this? — Get Off My Lawn, Sep 25 '18 at 15:35
There is no array, I want to be able to produce an array based on the keys (fields) defined on the type. — MaYaN, Sep 25 '18 at 15:36
@MaYaN Could you please explain your post in more detail with example? I am not able to understand it currently. — vibhor1997a, Sep 25 '18 at 15:44

Get Off My Lawn · Accepted Answer · 2018-09-25T15:56:07.167

7

You can use Reflect.construct() to get the keys, then use Object.keys() to convert that to an array.

Note: if the key doesn't have a default it won't be generated as you can see with four.

class Foo {
  constructor() {
    this.one = ''
    this.two = ''
    this.three = ''
    this.four
  }
}

console.log(Object.keys(Reflect.construct(Foo, [])))

edited Sep 25 '18 at 15:56

answered Sep 25 '18 at 15:50

Get Off My Lawn

34,175
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Exactly what I was looking for. Thanks!, Actually I don't need to initialize the fields either, it works beautifully. – MaYaN Sep 25 '18 at 15:56
Note that this isn't compatible with IE if you are targeting IE browsers. – Get Off My Lawn Sep 25 '18 at 15:58
6

Isn't it basically same as `new` though? – Nima Hakimi Sep 25 '18 at 15:58
This looks like a typescript version of C# reflection – Niladri Sep 25 '18 at 16:01
@NimaHakimi yes it is. " It is equivalent to calling new target(...args). ", the return is an instance of the target. There is no difference in this case. – rlemon Sep 25 '18 at 16:01

score 1 · Answer 2 · answered Sep 25 '18 at 15:44

1

If you just one to list the field's name from a class you can use the following:

let fields = Object.keys(myObj) as Array<MyClass>;

Hope it helps.

answered Sep 25 '18 at 15:44

Jose Anibal Rodriguez

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1
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17

Sajeetharan · Answer 3 · 2018-09-25T15:48:28.893

1

You need to instantiate const a = new Foo(); and access using Object.keys

class Foo {
    constructor(
        private one: string, 
        private two: string, 
        private three: string) {
    }
}

const a = new Foo();
console.log(Object.keys(a)); // ["prop"]

DEMO

EDIT: If you want to get from the type it is not possible since types won't be available once the code is compiled.

edited Sep 25 '18 at 15:48

answered Sep 25 '18 at 15:45

Sajeetharan

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1

As I explained in the post, I need to get this from the type and not an instance. – MaYaN Sep 25 '18 at 15:45
1

You cannot create from a type since all the types are gone once compiled. – Get Off My Lawn Sep 25 '18 at 15:47
I know how I can make a type based on another type's keys e.g. `const myType: keyof Foo` which would be: `one | two | three` but I need to turn that into an array of `['one', 'two', 'three']` – MaYaN Sep 25 '18 at 15:48
whats the problem of instantiating it? – Sajeetharan Sep 25 '18 at 15:49

score 1 · Answer 4 · answered Sep 25 '18 at 15:46

Since the code above will be transpiled to following js code:

var Foo = (function () {
    function Foo(one, two, three) {
        this.one = one;
        this.two = two;
        this.three = three;
    }
    return Foo;
}());

I'd first extract the constructor using const c = Foo.prototype.constructor and the get the name of arguments from that. This thread shows how to do that.

score 0 · Answer 5 · answered Sep 25 '18 at 15:30

0

Scan the Object.keys(Foo.prototype)

answered Sep 25 '18 at 15:30

eavichay

507
2
7

2

`Object.keys(Foo.prototype)` returns an empty array. – MaYaN Sep 25 '18 at 15:33
You need to initialize the values with a default, such as `null`. otherwise they won't show. – Get Off My Lawn Sep 25 '18 at 15:40
@GetOffMyLawn It is still empty. – MaYaN Sep 25 '18 at 15:42
use `new` as well `console.log(Object.keys(new Foo))` – Get Off My Lawn Sep 25 '18 at 15:46

How can I get properties of a type as an array in Typescript?

5 Answers5