Repository navigation
Maintain string litteral types when used as index signature type #17784
Description
TypeScript Version: 2.4.0
Code
function makeObj(foo: 'FOO') {
return { [foo]: 1 };
}
makeObj('FOO') // return type: `{ [x: string]: number}`Expected behavior:
Index signatures should have narrower types. Return type should be { [x: 'FOO']: number}
Actual behavior:
return type is: { [x: string]: number}
Use case
When making forms in React components I often use a method that returns an onChange method to be used on input onChange properties:
class Component<S> {
state: S;
setState(partialState: Partial<S>) { }
onFiledChanged = (filedName: keyof S) => (value) => {
this.setState({ [filedName]: value });
}
render() {
return <form>
<input onChange={ this.onFiledChanged('foo') } />
<input onChange={ this.onFiledChanged('bar') } />
</form>
}
}TypeScript fails to understand filedName is type 'foo' | 'bar'
Activity
mhegazy commentedon Aug 22, 2017
looks like a duplicate of #16687
mhegazy commentedon Sep 6, 2017
Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.