如何使用隐式键将对象文字声明为记录

Question

If a constant object literal is created without declaring a type, it will infer the keys from the value:

const OBJ = {foo: 42, bar: 42}

This will become a Record<'foo' | 'bar', number> Record<'foo' | 'bar', number> , which means IDEs will be helpful to offer auto-completion when typing OBJ.fo etc.

If the same object is declared like this:

const OBJ: Record<string, number> = {foo: 42, bar: 42}

then IDEs will pretend not to know that OBJ has keys foo and bar , since OBJ is a record of any string key. :(

Now, if the values are simple primitives, omitting the type and relying on implicit inferred types usually works fine, but if the record is more complex, such as a Record<string, MyEnum> or Record<string, (x: string, y: number) => number> etc, doing so will lose the type information of the values :

enum MyEnum {
  A = 42,
  B = 85
}

const OBJ = {foo: A, bar: B};

Now OBJ will be a Record<'foo' | 'bar', number> Record<'foo' | 'bar', number> , not a Record<'foo' | 'bar', MyEnum> Record<'foo' | 'bar', MyEnum> .

What I want is to declare a dictionary where the type of the values is given in the declaration, but keys are inferred.

The only ways I've found are:

1. Type each value

const OBJ = {foo: <MyEnum>A, bar: <MyEnum>B};

2. Declare keys separately

type Keys = 'foo' | 'bar';
const OBJ: Record<Keys, MyEnum> = {foo: A, bar: B};

Is there a better way, where the keys are inferred by the literal but the values are declared on the entire Record ?

Answer 1

I am not sure which TypeScript version you are using. Also, your example has an error, you need to use MyEnum.A instead of just A .

Let's look at your example:

enum MyEnum {
  A = 42,
  B = 85
}

const OBJ = {foo: MyEnum.A, bar: MyEnum.B};

OBJ has the correct type: { foo: MyEnum; bar: MyEnum; } { foo: MyEnum; bar: MyEnum; } { foo: MyEnum; bar: MyEnum; } . Tested in 3.9 > typescript.

Try on TS Playground

---

Regarding your requirements:

What I want is to declare a dictionary where the type of the values is given in the declaration, but keys are inferred.

You can use Generics function to infer type:

enum MyEnum {
  A = 42,
  B = 85
}

function fixType<T extends string>(value: Record<T, MyEnum>): Record<T, MyEnum> {
  return value
}

const OBJ = fixType({foo: MyEnum.A, bar: MyEnum.B})

And now OBJ has Record<"foo" | "bar", MyEnum> Record<"foo" | "bar", MyEnum> type.

Try on TS Playground

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If you are using Enum , you may be interested in this issue . To sum up, even if you specified which numbers your enum supports, TS still allows assigning any number...