Support custom typeof functions

Search Terms

• custom typeof
• custom typeof function
• custom type guard

Suggestion

When using a custom typeof‑like function, I’d like TypeScript compiler to be able to infer the correct type in the scope guarded by the custom typeof‑like function.

Use Cases

I need this to provide proper type information for the type function from Blissfuljs and similar projects.

The current approach requires defining the type(…) function as type(obj: any): string and then doing a type cast every time something is accessed within the if block:

/// <reference types="blissfuljs"/>
declare var something: any;
if ($.type(something) === "array") {
	(something as any[]).forEach(v => {/* stuff */})
}

Examples

/**
 * @param obj The variable to check the type of.
 * @return The result of `typeof obj` or the class name in lowercase for objects.
 *         In the case of numbers, if the value is `NaN`, then the result is `nan`.
 */
declare function type(obj: null): "null";
declare function type(obj: undefined): "undefined";

// This is to ensure that the type system short-circuits when it encounters primitive types.
declare function type(obj: number): "number" | "nan";
declare function type(obj: string): "string";
declare function type(obj: symbol): "symbol";
declare function type(obj: boolean): "boolean";

// Needed to ensure proper return values when wrapper objects are used.
/* tslint:disable:ban-types */
declare function type(obj: number | Number): "number" | "nan";
declare function type(obj: string | String): "string";
declare function type(obj: symbol | Symbol): "symbol";
declare function type(obj: boolean | Boolean): "boolean";
declare function type(obj: Function): "function";
/* tslint:enable:ban-types */

declare function type(obj: any[]): "array";
declare function type(obj: RegExp): "regexp";
declare function type(obj: any): string;
export = type;

example.ts

import type = require("./type-func");
declare var something: any;

if (type(something) === "array") {
	// $ExpectType any[]
	something;
} else if (type(somthing) === "number") {
	// $ExpectType number
	something;
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Bắt đầu với các khai báo trong type-func.d.ts và phần tái hiện trong example.ts ở issue, sau đó kiểm tra cách TypeScript xử lý narrowing đối với các phép kiểm tra typeof và các type guard do người dùng định nghĩa. Được xem là hoàn tất khi các phép so sánh được nêu suy luận ra any[] và number trong các nhánh tương ứng mà không cần cast, đồng thời giữ nguyên output và hành vi JavaScript hiện có.