Why does typescript define the type of the key as string when iterating over the keys of an object using a for loop?

Question

typescript

const object: IObject = {
    number: 0
}

for (let key in object) {
    object[key] =  Math.random();
}

Error log tsc:
Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'IObject '.
No index signature with a parameter of type 'string' was found on type 'IObject '.

Answer 1

Well, yes, it's a string. You need to instantiate key so that it contains the keys of the iterated object:

const object: IObject = {
    number: 0,
    superNumber: 0,
}

let key: keyof IObject;  // "number" | "superNumber"
for (key in object) {
    object[key] =  Math.random();
}