How to declare a custom class in google scripts?

Update:

As of spring 2020 Google has introduced a new runtime for Apps Script which supports Classes.
New scripts use this runtime by default, while older scripts must be converted. A prompt is displayed in the script editor, from which you can convert your scripts.

// V8 runtime
class Rectangle {
  constructor(width, height) { // class constructor
    this.width = width;
    this.height = height;
  }

  logToConsole() { // class method
    console.log(`Rectangle(width=${this.width}, height=${this.height})`);
  }
}

const r = new Rectangle(10, 20);
r.logToConsole();  // Outputs Rectangle(width=10, height=20)

Original (old) answer:

Historically Javascript is a “classless” language, classes are a newer feature which haven’t been widely adopted yet, and apparently are not yet supported by Apps Script.

Here’s an example of how you can imitate class behaviour in Apps Script:

var Polygon = function(height, width){
  this.height = height;
  this.width = width;
  
  this.logDimension = function(){
    Logger.log(this.height);
    Logger.log(this.width);
  }
};

function testPoly(){
  var poly1 = new Polygon(1,2);
  var poly2 = new Polygon(3,4);
  
  Logger.log(poly1);
  Logger.log(poly2);
  poly2.logDimension();
}