Can you set a object type to custom name instead of function in Javascript

Question

I was wondering if the following situation is possible. I am trying to write a object factory, that will construct different types of objects. What I am trying to figure out, is if it is possible in java script to set the type as something other then a function or object? Similar to in C# how a class instance of Car is a typeof Car, even though the base type is object. Hopefully I am explaining this clearly. The best way to explain it, is demonstrated below:

var object = function(){

switch(id){
    case 1:
        $_objInstance = new Car();
    break;
  case 2: 
    $_objInstance = new Animal();
    break;

 return $_objInstance;
}


function Car(){
    this.Name = "GM";
}


document.write(object instanceof Car);
or
typof(

object) == Car //is this possible?

Thanks in advance! Happy coding:)

you can't introduce new reflections to typeof, but you can specify a constructor for instanceoftargeting — dandavis
– dandavis, Commented Jan 27, 2016 at 17:29

T.J. Crowder · Accepted Answer · 2020-07-08 14:02:37Z

1

What I am trying to figure out, is if it is possible in java script to set the type as something other then a function or object?

No, you can't. However:

JavaScript does have an instanceof operator you can use to test an instance to see if it's likely to have been constructed by a given constructor:
```
  if (obj instanceof Car) {
      // ...
  }
```
Function#name remains unreliable in the wild (in that JavaScript engines don't set it reliably yet), you but you set it explicitly on your constructor functions:
```
  Car.name = "Car";
```

...and then use obj.constructor.name to find the name of the constructor. That relies on the object inheriting the constructor property correctly, which it will if you don't mess up the default object on the function's prototype property.

Several years back I wrote a blog post about typeof, instanceof, Object.prototype.toString, and various other ways you can use to determine what something is, although I left off that option of just setting your own name.

Here's an example of using .constructor.name:

function Car() {
}
Car.name = "Car";

function Animal() {
}
Animal.name = "Animal";

console.log("Randomly constructing 10 Cars or Animals:");
for (var n = 0; n < 10; ++n) {
    var x = Math.random() < 0.5 ? new Car() : new Animal();
    console.log("We got: " + x.constructor.name);
}

When I said above

That relies on the object inheriting the constructor property correctly, which it will if you don't mess up the default object on the function's prototype property.

let me give you an example that messes it up, so you know what to keep an eye out for. You routinely see people doing this:

// PROBLEMATIC: Wipes out the `constructor` property
function Car() {
}
Car.prototype = {
    drive: function() {
    }
    // ...
};

That is, rather than augmenting the object that Car.prototype got originally, they replace it. I prefer to augment (e.g., don't replace the whole object, just add properties to it), for various reasons, but if you do replace it you can just make sure you give the new one the correct constructor:

// Keeps `constructor` correct
function Car() {
}
Car.prototype = {
    constructor: Car,
    drive: function() {
    }
    // ...
};

edited Jul 8, 2020 at 14:02

answered Jan 27, 2016 at 17:35

T.J. Crowder

1.1m201 gold badges2k silver badges2k bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Casey ScriptFu Pharr Over a year ago

Thanks, an apologies for the delayed response. I was thinking about using the instance of. I do like what you explained above though about the "constructor.name". Now, I love and am a huge fan of Javascript. My main work is with this and also c#. I have to admit though, I worked a lot in jQuery, and never used the prototype. Should I get more into this if I plan on writing more extensive and heavier Javascript code? Meaning real projects, not just using it for regular web applications.

Casey ScriptFu Pharr Over a year ago

Thanks also for the example explaining what to watch for.

T.J. Crowder Over a year ago

@Casey: The useful thing about prototypes is that they let you share functionality across all instances created with the constructor function. They also let you add functionality (by adding to the prototype), which shows up on all instances (even ones already created) because of the way prototypical inheritance works (instances have a live link back to the prototype object). Whether you need that will depend on the kinds of projects you're doing.

Collectives™ on Stack Overflow

Can you set a object type to custom name instead of function in Javascript

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related