Specify T when you have "typeof T" in JSDOC, and in VsCode

Question

I've distilled an essence of my problem with following codes:

full source

I have Base class, and Derived, Derived2:

class Base {
    static get type() {
        return 'Base';
    }
}

class Derived extends Base {
}

class Derived2 extends Base {
}

Now I have variable t, which could be an instance of Derived or Derived2. It can also be changed multiple times during runtime.

/** @type {Base} */
var t = new Derived();
//or
var t = new Derived2();

我有一个函数检查t是否是已传递类的实例，如果它是已传递类的实例或未定义，则返回t。

/**
 * @template {typeof Base} T
 * @param {T} cl
 * @returns {T}  /// <-- I can't figure out how o return an instance of T
 */
function checkTop( cl ) {
    if ( t instanceof cl ) {
        return t;
    }
    return undefined;
}

当我调用checkTop（Derived）时，其返回类型应该为Derived。但是在上面的jsdoc中，其返回类型为'typeof Derived'。但我想将返回类型设为“派生”。

let d1 = checkTop( Derived ); // 'typeof Derived', but I want 'Derived' as return type

同样，d2被识别为“ typeof Derived2”

let d2 = checkTop( Derived2 ); // 'typeof Derived2'.. but I want 'Derived2' as return type

如何在JSDOC中指定返回类型，以使checkTop( Derived );返回类型为Derived，而checkTop( Derived2 )返回类型为'Derived2'。

我尝试以下返回类型：

/**
 * @template {Base} B 
 * @template {typeof B} T
 * @param {T} cl
 * @returns {B}
 */
function checkTop( cl )

和

/**
 * @template {typeof Base} T
 * @param {T} cl
 * @returns {instanceof T}
 */
function checkTop( cl )

如果在JSDOC中是不可能的，但在打字稿中是不可能的，那也将有所帮助，但我更喜欢JSDOC解决方案。

Answer 1

将模板定义为您需要返回的类型，并将参数定义为该类型的构造函数

/**
* @template {Base} T
* @param {new T} cl
* @returns {T}
*/
function checkTop( cl ) {
    if ( t instanceof cl ) {
        return t;
    }
    return undefined;
}

结果将是：

function checkTop<T extends Base>(cl: new () => T): T