在Javascript中，函数声明是创建构造函数的唯一方法吗？

Question

In JavaScript, is function declaration the only way to create a constructor that can be used to instantiate a new object. If so, Why ? This question arises from the following code.

var customevent = {
        prop1 : "div",
        prop2 : "div2"
    }

    var myevent_obj = new customevent();

Browser JS console show

SyntaxError: customevent is not a constructor.

Answer 1

By definition, a constructor is an object that implements the internal [[call]] and [[Construct]] methods. Except for a few built–in functions, all native functions are constructors. Most host functions are not constructors, though some are.

EMCAScript 2015 also defines exotic function objects, which might be built–in but need not be constructors.

There are two methods for creating a native function that can be a constructor:

1. Function declaration: function MyConstructor(){...}
2. Function expression (usually through assignment): var MyConstructor = function(){...};

That's it.

In the OP:

var customevent = {
        prop1 : "div",
        prop2 : "div2"
    }

customevent is an Object, not a Function. Plain objects don't implement either [[Call]] or [[Construct]] internal methods, therefore they can't be constructors. They can be used as prototypes though:

function customConstructor(){}
customConstructor.prototoype = customevent;

or

var newCustomEvent = Object.create(customevent);