It’s just to properly reset the constructor property to accurately reflect the function used to construct the object. Sub.prototype = new Super(); console.log(new Sub().constructor == Sub); // -> ‘false’ Sub.prototype.constructor = Sub; console.log(new Sub().constructor == Sub); // -> ‘true’
It is a tricky thing indeed, more complicated than it should be if the language was designed well… Basically, you just can’t do it cleanly in current versions. Objects other than functions can not be callable. In future Javascript versions, you can do it with a “proxy” object that can define a “call” handler. But … Read more
Defining a function with whatever = function() { … } tends to create what’s called a “closure”, where the function can access local variables of the function that defines it. When you say this.fn = function() { … }, each object gets an instance of the function (and a new closure). This is often used … Read more
You can achieve desired outcome from the other end by making the prototype methods not enumerable: Object.defineProperty(Array.prototype, “containsKey”, { enumerable: false, value: function(obj) { for(var key in this) if (key == obj) return true; return false; } }); This usually works better if you have control over method definitions, and in particular if you have … Read more
You cannot change a constructor by reassigning to prototype.constructor What is happening is that Rabbit.prototype.constructor is a pointer to the original constructor (function Rabbit(){…}), so that users of the ‘class’ can detect the constructor from an instance. Therefore, when you try to do: Rabbit.prototype.constructor = function Rabbit() { this.jumps = “no”; }; You’re only going … Read more
It is a function with a prototype that takes two scalar arguments. There are strong arguments for not actually using Perl prototypes in general – as noted in the comments below. The strongest argument is probably: Far More Than Everything You’ve Ever Wanted to Know about Prototypes in Perl There’s a discussion on StackOverflow from … Read more
Simply because this is the way JSON works. From the ES5 spec: Let K be an internal List of Strings consisting of the names of all the own properties of value whose [[Enumerable]] attribute is true. This makes sense, because there is no mechanism in the JSON specification for preserving information that would be required … Read more
Because in a method call the this argument is always (in sloppy mode) casted to an object. What you see is a String object, which was produced from the “test” primitive string value. The array on which you call your method is already an object, so nothing happens and you just get the array as … Read more
The reason is that the ES5 spec says so: The Function prototype object is itself a Function object (its [[Class]] is “Function”) that, when invoked, accepts any arguments and returns undefined. Note it’s common in ES5 to make the prototype of some class a member of that class: Object.prototype is an Object object. Function.prototype is … Read more
The prototype object is meant to be used on constructor functions, basically functions that will be called using the new operator to create new object instances. Functions in JavaScript are first-class objects, which means you can add members to them and treat them just like ordinary objects: var STORE = { item : function() { … Read more