The Typescript compiler prohibits calling methods on a union of two array types

Question

The Typescript compiler prohibits calling methods on a union of two array types

Take a look at this function:

function getArray(): string[] | number[] {
    return [];
}

To keep it short, when I execute the following code:

getArray().forEach(item => console.log(item));

I encounter this error from the compiler:

TS2349 Cannot invoke expression whose type lacks a call signature.

Example on TypeScript Playground for reference

What could be causing this issue? From my perspective, it seems like forEach should work without any problems because the function definitely returns an array. However, the IntelliSense suggests the method, but when selected, the error appears in the output.

Is this a bug or am I overlooking something simple?

Edit: Although there are some alternatives like returning Array<string | number> or using overloads, I'm specifically interested in understanding why returning a union of two array types prevents method invocation.

We can confirm that the returned type is an array containing elements of type string | number.

arrays typescript types

Answer 1

Answer №1

When it comes to processing method calls of union types with different signatures, there isn't a straightforward "safe" way to handle them as if they were all the same. TypeScript does allow method calls when the methods have identical signatures, but that's not always the case.

For example, consider the following scenario:

class ValueHaver<T> {
    value: T;
    update(x: () => T) { this.value = x(); }
}

let x: ValueHaver<number> | ValueHaver<string> = /*...*/;
x.update(() => 42);

In this code snippet, if x ends up being a ValueHaver<string>, you'll have a number in place of a string, which is incorrect.

Handling this kind of method call is just as tricky as dealing with a forEach invocation on an

Array<number> | Array<string>

.

One might suggest iterating through each possible type in the union and invoking the corresponding method X.y. This approach theoretically could work, but practically speaking (especially in TypeScript 2.0), unions can have numerous constituents making it impractical to predict which specific type will work without thorough type checking every time, including its arguments due to contextual typing. Unfortunately, constantly rechecking the entire call and its arguments would be too expensive computationally. Additionally, the TS compiler caches resolved expression types, limiting its ability to dynamically adjust to these changing contexts.

Answer 2

When it comes to processing method calls of union types with different signatures, there isn't a straightforward "safe" way to handle them as if they were all the same. TypeScript does allow method calls when the methods have identical signatures, but that's not always the case.

For example, consider the following scenario:

class ValueHaver<T> {
    value: T;
    update(x: () => T) { this.value = x(); }
}

let x: ValueHaver<number> | ValueHaver<string> = /*...*/;
x.update(() => 42);

In this code snippet, if x ends up being a ValueHaver<string>, you'll have a number in place of a string, which is incorrect.

Handling this kind of method call is just as tricky as dealing with a forEach invocation on an

Array<number> | Array<string>

.

One might suggest iterating through each possible type in the union and invoking the corresponding method X.y. This approach theoretically could work, but practically speaking (especially in TypeScript 2.0), unions can have numerous constituents making it impractical to predict which specific type will work without thorough type checking every time, including its arguments due to contextual typing. Unfortunately, constantly rechecking the entire call and its arguments would be too expensive computationally. Additionally, the TS compiler caches resolved expression types, limiting its ability to dynamically adjust to these changing contexts.

Answer 3

Answer №2

You are unable to utilize the forEach method on the type string[] | number[].

A more suitable approach would be to use: Array<string | number>:

function createNewArray(): Array<string | number> {
    return [];
}

var result = createNewArray().forEach(element => { console.log(element) }); // no errors

If you prefer, you can also cast to one of the types:

(createNewArray() as string[]).forEach(item => { /* item is a string */ });

Alternatively, you can cast to any:

(createNewArray() as any[]).forEach(item => { /* item is any */ });

Answer 4

You are unable to utilize the forEach method on the type string[] | number[].

A more suitable approach would be to use: Array<string | number>:

function createNewArray(): Array<string | number> {
    return [];
}

var result = createNewArray().forEach(element => { console.log(element) }); // no errors

If you prefer, you can also cast to one of the types:

(createNewArray() as string[]).forEach(item => { /* item is a string */ });

Alternatively, you can cast to any:

(createNewArray() as any[]).forEach(item => { /* item is any */ });

The Typescript compiler prohibits calling methods on a union of two array types

Answer №1

Answer №2

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