如何在javascript中检查目标是否为“类”

Question

Since ECMAScript 2015, jvascript start to support class declaration. But there is no way to distinguish between 'class declaration' and 'function declaration'.

class yesClass {
  //It is possible to omit defining constructor
  constructor () {}
}
typeof yesClss; //"function"
yesClss instanceof Function; //true
yesClss instanceof Object; //true
yesClass.constructor.name; //"Function"

There is no way to distinguish them?

Answer 1

In javascript, class declaration requires 'new' operator to use a constructor, which exist or not. So call class without 'new' operator occurs TypeError.

class yesClass {}
yesClass();
//Uncaught TypeError: Class constructor yesClass cannot be invoked without 'new'

This "TypeError" can be caught with "try ... catch ...", like following:

try {
  yesClass();
} catch ( e ) {
  //Case of user-defined class, "Class constructor ..."
  //Case of native function which behaves like class, Document, "Failed to construct ..."
  console.log( e.message.match( /^Class constructor/ ) !== null || e.message.match( /^Failed to construct/ ) !== null );
}

Heading code let you know that is "class declaration" or not.

The other simple and limited way, specified in a REAL class declaration, is using toString method. When the target is user-defined class, the result of toString method starts with "class".

yesClass.toString();
//"class yesClass {}"

The Object has the top hierarchy in javascript and toString method is a method of Object. Therefore every object in javascript has toString method except several cases which are some native objects and manually removed objects.

Test following "JS Bin" archive to test heading context. In the following link, I wrote a function, which named as "isClass". This function returns boolean or object, differs by parameters. https://jsbin.com/tekeyu/edit?js,console