What does it mean `Index signature of object type implicitly has an 'any' type.`

Question

I have the following code and noImplicitAny:true in tsconfig:

let o = {a: 3};

// works fine
o['a'] = 3;

// reports an error 
// Error:(4, 1) TS7017:Index signature of object type implicitly has an 'any' type.
o['b'] = 3;

What does this error mean?

Here it is on the TypeScript playground - be sure to click Options and set noImplicitAny (doesn't seem to remember options in shareable links).

Answer 1

The error is caused because the index signature is not explicitly defined.

You can declare an index signature explicitly like so:

let ox : { [index:string] : number } = {};
ox['b'] = 3; 

The reason o['a'] = 3; doesn't error is because of rule 1 of bracket notation property access which is defined in the following rules from the spec:

1. If index is a string literal or a numeric literal and object has an apparent property (section 3.11.1) with the name given by that literal (converted to its string representation in the case of a numeric literal), the property access is of the type of that property.
2. Otherwise, if object has an apparent numeric index signature and index is of type Any, the Number primitive type, or an enum type, the property access is of the type of that index signature.
3. Otherwise, if object has an apparent string index signature and index is of type Any, the String or Number primitive type, or an enum type, the property access is of the type of that index signature.
4. Otherwise, if index is of type Any, the String or Number primitive type, or an enum type, the property access is of type Any.
5. Otherwise, the property access is invalid and a compile-time error occurs.