CODEWITHSUNDEEP
typescripttypescript-generics
sidoshi
sidoshi

Reputation: 2160

Is there a way to constraint a generic type to plain objects only in typescript?

Using typescript, is there a way to constraint a generic type to allow values of type object only? Not array and other types.

For eg:

function check<O extends object>(ob: O): O {
  return ob
}

check({}) // should be fine
check([]) // should show error, but does not.

A possible real world use case:

function extend<A extends object, B extends object>(a: A, b: B): A & B {
  return {
    ...a,
    ...b,
  };
}

// This is fine, return type is { x: number } & { y: string }
extend({x: 5}, {y: "str"})

// This should not be allowed
// Return type is number[] & string[]
// Accessing a property yields impossible types like number & string
extend([1,2,3], ["s", "i", "d"])

// More examples for variants which I don't want to allow
extend({}, function() {})
extend({}, new Map())

Upvotes: 12

Views: 4603

Answers (2)

Rubilmax
Rubilmax

Reputation: 41

In newer versions of Typescript, you can use object & Exclude<T, any[]> to represent an object that cannot be an array.

Upvotes: 4

Titian Cernicova-Dragomir
Titian Cernicova-Dragomir

Reputation: 250366

You can use a conditional type to add to the parameter of the parameter if O extends any[]. What you add can ensure that the call will be an error. I usually add string literal types and think of them as a custom error message:

function check<O extends object>(ob: O & (O extends any[] ? "NO Arrays !" : {})): O
function check<O extends object>(ob: O): O {
  return ob
}

check({}) // should be fine
check([]) // error

Upvotes: 6

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