CODEWITHSUNDEEP
typescript
pwunderlich
pwunderlich

Reputation: 84

Arrays of a specific type in typescript

I define an array of a specific type in typescript. When assigning values that do not correspond to the type, error messages are generated as required. An assignment of the type any works, even if the type is not correct.

Do I misunderstand the type definition of an array? Or do I underestimate the "power" of the anys :-)

Here is a brief example:

export class ItemList {

  items: Array<string> = [];

  constructor() {

    // push a string directly => works
    this.items.push('item 1');

    // push a string variable => works
    let item2:string = 'item 2';
    this.items.push(item2);

    // push a number variable => doesn't work
    let item3 = 3;
    this.items.push(item3);

    // push a number as any type => works
    let item4:any = 4;
    this.items.push(item4);

  }

}

let itemList = new ItemList();

The error from the tsc is:

error TS2345: Argument of type 'number' is not assignable to parameter of type 'string'.

Funny thing: the plunkers here works.

Upvotes: 5

Views: 21667

Answers (3)

Konstantin Pelepelin
Konstantin Pelepelin

Reputation: 1562

do I underestimate the "power" of the anys

Yes.

any is assignable to any type, so even though items is string[] and items.push() accepts only string, any is accepted.

But in the end I do not have the result that I expect.

Disallow any then. There are a noImplicitAny rule for tsconfig and no-any and no-unsafe-any rules for tslint.

Still, as seen in @artem's example, since TypeScript does not control co- and contravariance of function parameters, its type system can't guarantee the desired type safety.

Upvotes: 0

Rahul Uttarkar
Rahul Uttarkar

Reputation: 3645

Array of specific type in typescript

export class RegisterFormComponent 
{
     genders = new Array<GenderType>();

     loadGenders()
     {
            this.genders.push({name: "Male",isoCode: 1});
            this.genders.push({name: "FeMale",isoCode: 2});
     }
}

type GenderType = { name: string, isoCode: number };    // Specified format

Array of any type in typescript

export class RegisterFormComponent 
{
     genders = new Array<any>();          
}

Upvotes: 2

Tumen_t
Tumen_t

Reputation: 801

What you are looking for is union types.

Try this 

items: (string|number)[]; // or...
items: Array<string|number>;

Or you can initialize it like this 

items: (string|number)[] = ["Hello", 154]; // or...
items: Array<string|number> = ["Hello", 154];

Upvotes: 17

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