Question

This is a newbie query. I went through few similar posts, but they are not helping me enough. This post has 2 queries, but am putting together because their root seems the same.

I came across the following code snippet :

var Server = module.exports.Server = function Server(listener) {
  if (!(this instanceof Server)) return new Server(listener);
  //other code
}

module.exports.createServer = function(listener) {
  return new Server(listener);
};

I am unable to understand the use of if (!(this instanceof Server)) ; when can this not point to Server here ?

I tried putting a quick test for this :

var createTest = function(){
    console.log(this.toString());
    return new Test();
};

var Test = function Test(){
    console.log(this instanceof Test);
    console.log(this.toString());
    if (!(this instanceof Test)) 
    {
        return new Test();
    }
}

var tester = createTest();

which outputs :

[object global]
true
[object Object]

which further confuses me over why this.toString prints [object Object] – shouldn’t it be [object Test] ?

Thanks !

Answer 1

That’s an idiom used to make new optional. For example:

function TraditionalTest() {
}
function IdiomaticTest() {
    if(!(this instanceof IdiomaticTest)) return new IdiomaticTest();
}

console.log(new TraditionalTest());  // [object Object]
console.log(TraditionalTest());      // undefined
console.log(new IdiomaticTest());    // [object Object]
console.log(IdiomaticTest());        // [object Object]

---

As for why it’s [object Object] rather than [object Test], I don’t know why it’s defined that way, but that’s the way it’s defined:

When the toString method is called, the following steps are taken:

1. If the this value is undefined, return “[object Undefined]“.
2. If the this value is null, return “[object Null]“.
3. Let O be the result of calling ToObject passing the this value as the argument.
4. Let class be the value of the [[Class]] internal property of O.
5. Return the String value that is the result of concatenating the three Strings “[object“, class, and “]“.

[[Class]] is referenced several times in the specification, but it only ever represents built-in types:

• Arguments
• Array
• Boolean
• Date
• Error
• Function
• JSON
• Math
• Number
• Object
• RegExp
• String