I'm trying to invert an object's fields if a value exists, but I'm getting some error that I cannot decipher.
interface Control {
name1: boolean;
name2: boolean;
...
}
const values = ['name1', 'name2', ...];
const control: Control = {
name1: false,
name2: false,
...
}
for (const value of values) {
if ((value in control) {
control[value] = !control[value]; // error
}
}
Error message:
Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'Control'.
No index signature with a parameter of type 'string' was found on type 'Control'.ts(7053)
However, the error goes away if I explicitly pass one of the object's fields, such as control['name1']
.
Is there something I am not understanding?
CodePudding user response:
TypeScript doesn't know that your values
array is an array of specific strings, only that it's a string[]
. Try this:
interface Control {
name1: boolean;
name2: boolean;
}
const values = ['name1', 'name2'] as const;
const control: Control = {
name1: false,
name2: false,
}
for (const value of values) {
if (value in control) {
control[value] = !control[value]; // error
}
}
That as const
tells TypeScript to specify the type of values
more tightly to be ['name1', 'name2']
in this case, so when you're iterating through it value
will be typed as 'name1' | 'name2'
and TypeScript will understand that those values can be used to index a Control
.
CodePudding user response:
The value
variable can be any value during application runtime and TypeScript has no awareness that it is a valid key defined within the control
object. Hence it cannot simply infer the value type of control[value]
.
The fix is however very simple, all that needs to be done is to tell TypeScript that the values
array only contains valid keys from the Control
type via the use of the keyof Control
syntax:
const values: (keyof Control)[] = ['name1', 'name2'];
Here is a link to the Playground.