CODEWITHSUNDEEP
typescript
Andor Polgar
Andor Polgar

Reputation: 501

Best way to iterate over objects of different types

I have an array that contains objects of different types. I learned from the documentation that TypeScript can deduce union types such as string | number and number | number[] based on the code structure, but it can't seem to deal with more complex interfaces.

interface A {
    type: string;
    a: string;
}

interface B {
    type: string;
    b: string;
}

const a: A = { type: 'a', a: 'foo'};
const b: B = { type: 'b', b: 'bar'};

const arr = [a, b];

arr.forEach((item) => {
    if (item.type === 'a') {
        console.log(item.a); // Property 'a' does not exist on type 'A | B'.
    }
    else if (item.type === 'b') {
        console.log(item.b); // Property 'b' does not exist on type 'A | B'.
    }
});

What's the best practice in this case? Is it okay to just cast the item to the expected type in each block?

Upvotes: 0

Views: 331

Answers (2)

Jad Al-Hamwi
Jad Al-Hamwi

Reputation: 169

To improve that a little bit more, you can add to @Evert solution the following:

enum Types{
a,
b}
interface A{
type:Types.a;
 a:string
}
interface B{
type:Types.b
b:string
}

This can be useful to avoid spelling mistakes.

Upvotes: 1

Evert
Evert

Reputation: 99581

Is it possible you just need to adjust your types a bit:

interface A {
    type: 'a';
    a: string;
}

interface B {
    type: 'b';
    b: string;
}

Upvotes: 3

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