"typeof keyof T" - Getting types of generic object's keys (Make keyof returning value instead of type)

[lsagetlethias]

Search Terms

• typeof keyof
• "typeof keyof"
• typeof key in object
• typeof keyof object

More or less similar issue: #23955

Suggestion

When using a keyof on a type, it returns a list of specific type of each key name.
It could be useful to get the type of the value of each "keyof type".

Use Cases

For reflexion functions, it could be useful to manipulate and typecheck dynamically generic class and type.

Examples

Playground link:
https://www.typescriptlang.org/play/#src=interface%20IModel%20%7B%0D%0A%20%20%20%20id%3A%20number%3B%0D%0A%7D%0D%0A%0D%0Afunction%20foo%3CT%20extends%20IModel%3E(source%3A%20T%2C%20value%3A%20typeof%20keyof%20T%2C%20property%3A%20keyof%20T)%20%7B%0D%0A%20%20%20%20source%5Bproperty%5D%20%3D%20value%3B%0D%0A%7D%0D%0A%0D%0Afoo(%7B%0D%0A%20%20%20%20id%3A%201%2C%0D%0A%7D%2C%20'bar'%2C%20'id')%3B

In this case, I would like when I use the foo() function, to have the second parameter to provide dynamically the type of values set by the third parameter (in ctrl+space)

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript / JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. new expression-level syntax)

[mhegazy]

here is the correct use of keyof:

function foo<T extends IModel, K extends keyof T>(source: T, value: T[K], property: K) {
    source[property] = value;
}

[lsagetlethias]

Wow thank you very much @mhegazy, quite complex.
Can you link me where it is in the documentation ?

[mhegazy]

http://www.typescriptlang.org/docs/handbook/advanced-types.html#index-types

[lsagetlethias]

Great, thank you. Sorry for missing that part.