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JavaScript Object-Oriented JavaScript (2015) Constructor Functions and Prototypes Creating Multiple Instances with Constructors

Posted by Adiv Abramson
Adiv Abramson
6,919 Points

How do you prevent an object from being instantiated if the call is syntactically valid?

For example, if we have a constructor function in Javascript (which to me seems to be the equivalent of a class definition in Java), how can we make the invocation fail intentionally if one or more parameters falls outside an accepted domain? Thanks!

function myObject(height, length, width) {
   if (isNaN(height) || isNaN(length) || isNaN(width)) {
      /*what do I write here so that a new instance of myObject 
         is NOT created? JS doesn't isn't strongly typed so an NaN
         error would not be caught at compile time */
   }// if (isNaN(height) || isNaN(length) || isNaN(width))


}//myObject

var myBadObject = new myObject("rtvc", false, Math.sqrt(2));

Perhaps this is a case where a constructor function should return a value to indicate success or failure?

3 Answers

Rich Donnellan
MOD
Rich Donnellan
Treehouse Moderator 27,696 Points
Rich Donnellan
Rich Donnellan
Treehouse Moderator 27,696 Points

Adiv,

Building off of Justin's answer, you'd be better suited using the typeof operator due to isNaN's ability to throw false positives. The AND (&&) logical operator must be using instead of OR (||), as it will return true if any case is true (not what we want here).

Notice the NOT (!) operator on the grouped if:

function Object(height, length, width) {
  if (!(typeof height === 'number' && typeof length === 'number' && typeof width === 'number')) {
    throw new Error("All parameters must be numbers.");
  } else {
    this.height = height;
    this.length = length;
    this.width = width;
    console.log("Congrats! These are all numbers.");
  }
}

//Test data
var badObject = new Object('rtvc', false, Math.sqrt(2));
//var goodObject = new Object(6, 12, Math.sqrt(2));

Hope this helps some.

– Rich

Justin Perry
Justin Perry
12,530 Points
Justin Perry
Justin Perry
12,530 Points

Adiv, you can throw an exception to stop the instantiation of the object.

function Object(height, length, width) {
   if (isNaN(height) || isNaN(length) || isNaN(width)) {
    throw new Error(" parameters must be numbers");

   } else {
     this.height = height;
     this.length = length
     this.width = width;

}//myObject
}
var myBadObject = new Object("rtvc", false, Math.sqrt(2));
Matija Komorčec
Matija Komorčec
13,123 Points
Matija Komorčec
Matija Komorčec
13,123 Points

Rich, we can use OR operator too, because it doesn't matter if we enter 1 parameter wrong, or all 3, we still want to throw an error. But the way you grouped the IF would work great.

Iain Simmons
Iain Simmons
Treehouse Moderator 32,305 Points

I'm trying to follow that logic in my head... if you use OR, then you need a ! before each part. Otherwise if you used OR within a group that has the ! on the outside, it would stop at the first true statement on the inside...

...right?