Keyof inferring string | number when key is only a string

Question

I define an AbstractModel like so:

export interface AbstractModel {
   [key: string]: any
}

Then I declare the type Keys:

export type Keys = keyof AbstractModel;

I would expect that anything with the Keys type would be interpreted univocally as a string, for example:

const test: Keys;
test.toLowercase(); // Error: Property 'toLowerCase' does not exist on type 'string | number'. Property 'toLowerCase' does not exist on type 'number'.

Is this a bug of Typescript (2.9.2), or am I missing something?

Answer 1

As defined in the Release Notes of TypeScript 2.9, if you keyof an interface with a string index signature it returns a union of string and number

Given an object type X, keyof X is resolved as follows:

If X contains a string index signature, keyof X is a union of string, number, and the literal types representing symbol-like properties, otherwise

If X contains a numeric index signature, keyof X is a union of number and the literal types representing string-like and symbol-like properties, otherwise

keyof X is a union of the literal types representing string-like, number-like, and symbol-like properties.

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This is because: JavaScript converts numbers to strings when indexing an object:

[..] when indexing with a number, JavaScript will actually convert that to a string before indexing into an object. That means that indexing with 100 (a number) is the same thing as indexing with "100" (a string), so the two need to be consistent.

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Example:

let abc: AbstractModel = {
    1: "one",
};

console.log(abc[1] === abc["1"]); // true

When you only want the string keys, then you could only extract the string keys from your interface like so:

type StringKeys = Extract<keyof AbstractModel, string>;

const test: StringKeys;
test.toLowerCase(); // no error

Also the TypeScript compiler provides an option to get the pre 2.9 behavior of keyof:

keyofStringsOnly (boolean) default false

Resolve keyof to string valued property names only (no numbers or symbols).

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