How to write custom type for array Typescript?

Question

I have interface type:

interface IForm {
   id: number;
   name: string;
}

I need to write custom type to store this object:

let a = ["one": [IForm, IForm, IForm], "two": [IForm, IForm, IForm]]

How to build this in TypeScript?

In result I need to get all forms by key?

let forms = a["one"]; --> [IForm, IForm, IForm]
forms[0]; --> IForm

you could check the type autogenerated by typescript for a suggestion. — toskv
– toskv, Commented Jun 27, 2018 at 11:00
What's with the squery braces in let a = [ ... ]. Do you mean curly braces let a = { ... }? — Sefe
– Sefe, Commented Jun 27, 2018 at 11:06

Sefe · Accepted Answer · 2018-06-27 11:21:50Z

1

You can use a mapped type:

type FormList = { [name: string]: IForm[]; };

Or in case "one" and "two" are fixed properties, simply:

interface FormList {
    one: IForm[];
    two: IForm[];
}

This definition implies that properties one and two are always present. If not, you can make them optional:

interface FormList {
    one?: IForm[];
    two?: IForm[];
}

edited Jun 27, 2018 at 11:21

answered Jun 27, 2018 at 11:06

Sefe

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6 Comments

POV Over a year ago

What is difference between these type? What is more priority?

POV Over a year ago

I tried fill object based this type: let a: FileList; a = { "profile": [{ id: 1, name: "ss" }}

Sefe Over a year ago

I don't understand the meaning of "priority" here. The difference is that with a mapped type you are free to use any key, not just "one" and "two". With an interface you specify that only "one" and "two" are allowed.

Sefe Over a year ago

You are missing a bracket: let a: FileList; a = { "profile": [{ id: 1, name: "ss" }]}

POV Over a year ago

It is still incorrect: type FormList = { [name: string]: IForm[]; }; let a: FileList; a = { "profile": [{ id: 1, name: "ss" }]}

|

Titian Cernicova-Dragomir · Accepted Answer · 2018-06-27 11:18:31Z

1

If the values are arrays of any length and the keys are fixed you can use an interface with those specific fields

 interface FormList [
     one: IForm[];
     two: IForm[];
 }

If the number of items in each array is always 3 you can use a tuple type [IForm, IForm, IForm]

 interface FormList [
     one: [IForm, IForm, IForm];
     two: [IForm, IForm, IForm];
 }

If you want to allow any key to be used you can use an index signature

 interface FormList {
     [key: string] : [IForm, IForm, IForm];
 }

edited Jun 27, 2018 at 11:18

answered Jun 27, 2018 at 11:17

Titian Cernicova-Dragomir

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3 Comments

POV Over a year ago

What is difference between type and interface definition? Is is possible to write using type?

Titian Cernicova-Dragomir Over a year ago

In this case there is not much difference, you can write using type as well type FormList = { [key: string] : [IForm, IForm, IForm]; }

POV Over a year ago

Mustumesc mult! :)

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