Keyof inferring string | number when key is only a string

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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I have faced similar problem. I resolved it by enforcing key to be string:

export type Keys = keyof AbstractModel & string;

Other option would be to convert key to string: test.toString().toLowercase()

For a generic typescript utility, you can use the following:

type KeyOf<T extends object> = Extract<keyof T, string>;

Usage:

const sym = Symbol();

const obj = {
  [sym]: true,
  foo: 'foobar',
  bar: 'barfoo',
  1: 'lorem'
}

let key: KeyOf<typeof obj> = 'foo'; // 'foo' | 'bar'

key = 'bar'; // ok
key = 'fool'; // error
key = 1; // error

playground