Question

var result = function(){}
console.log("constructor" in result);//true
console.log("__proto__" in result);//true
console.log("prototype" in result);//true

for (var prop in result) {
   console.log(prop); // no output!!
}

when I use in for determining if property in the result, it returns true; But when I use for-in, there is no property in the result, why?

Answer 1

ref: https://developer.mozilla.org/en/JavaScript/Reference/Statements/for…in

A for…in loop does not iterate over non–enumerable properties.
Objects created from built–in constructors like Array and Object have
inherited non–enumerable properties from Object.prototype and
String.prototype that are not enumerable, such as String’s indexOf
method or Object’s toString method. The loop will iterate over all
enumerable properties of the object or that it inherits from its
constructor’s proptotype (including any which overwrite built-in
properties).