Unable to 'index type' using a string variable

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?

>Solution :

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
  }
}

TypeScript Playground

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.