CODEWITHSUNDEEP
typescript
richflow
richflow

Reputation: 2144

Type 'X' is not assignable to type MyType[keyof MyType]

I'm having trouble with the following:

   interface MyType {
     [key: number]: number;
   }

   function myfunc<MyType>(map: MyType, key: keyof MyType) {
     map[key] = 4; // Error: type '4' is not assignable to type MyType[keyof MyType]
   }

I'm not sure what I need to do to make it safe to assign something to map[key]?

Upvotes: 1

Views: 49

Answers (2)

kmdreko
kmdreko

Reputation: 59942

The <MyType> on myfunc introduces a template parameter that is different from interface MyType. Just get rid of it and it'll work.

function myfunc(map: MyType, key: keyof MyType) {
  map[key] = 4;
}

Upvotes: 1

spender
spender

Reputation: 120410

In your function, you have (for unknown reasons) defined a generic type variable MyType which occludes your actual MyType declaration. This means that within the scope of the function, MyType refers to a generic type for which no further information can be inferred... hence the error.

It needs to change from

function myfunc<MyType>(map: MyType, key: keyof MyType){//...

to just

function myfunc(map: MyType, key: keyof MyType){//...

Upvotes: 1

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