What is the method for determining the type of a TypeScript class member that is associated with a commonly used symbol such as Symbol.toStringTag?

Does anyone know the correct TS syntax for extracting the type of a class method indexed with a well-known Symbol? Here are two incorrect methods:

type T = String[Symbol.toStringTag];
// 'Symbol' only refers to a type, but is being used as a namespace here.(2702)
// Exported type alias 'T' has or is using private name 'Symbol'.(4081)

type U = String[Symbol['toStringTag']];
// Property 'toStringTag' does not exist on type 'Symbol'.(2339)

Click here to play with the code

Non-well-known symbols are currently unsupported in TS 4.1 (Fix may be coming soon) but according to ES6 well-known symbols should be supported.

For more insights on resolving this issue, you can refer to this helpful thread on Stack Overflow.

typescripttypescript-typings

Answer №1

Utilizing the SymbolConstructor.toStringTag type can be challenging as it is simply a symbol. To overcome this limitation, you can employ a conditional type like so:

type ExtractToStringTag<T> = T extends { [Symbol.toStringTag]: infer V } ? V : never;

This code snippet checks if the object contains a defined [Symbol.toStringTag], infers its type, and returns it. If not, it defaults to never. For example:

type Result = ExtractToStringTag<Int16Array>

will result in the literal string type "Int16Array". However, when applied to :

type Result = ExtractToStringTag<String>

it will yield never because the current lib.d.ts does not define [Symbol.toStringTag] for String.

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