Return JavaScript class value instead of object reference (in ES6)

Question

I'm looking to create a class so that it will return a value instead of the class object in ES6.

It's pretty much the same question as this but in ES6. This touches on it a little bit, but you still have to explicitly call a property.

I want to do something like:

class Foo {
  constructor(value) {
    this.value = value;  // returns an object other than *this*.
  }
}    

var foo = new Foo(4);
console.log(foo) // 4

But currently it returns {value: 4}.

Why would you want to do something like this? That doesn't seem to make sense. What problem are you actually trying to solve? — Felix Kling
– Felix Kling, Commented Jun 22, 2015 at 0:09
foo is an object, not a primitive type, if you need to get just a value use a regular var value — kainlite
– kainlite, Commented Jun 22, 2015 at 0:11
If you don't want to create something that looks like a class, just use a function that returns a value. Why create a class if you don't want a class! — Robert Moskal
– Robert Moskal, Commented Jun 22, 2015 at 0:11
I'm creating a currency class. So it needs to retain a float value and return a string value. For example, 4.5689 and "$4.57". I've already created a library to do this in with helper functions. I just wanted to take ES6 classes for a test drive. :) — gojohnnygo
– gojohnnygo, Commented Jun 25, 2015 at 5:15

score 10 · Accepted Answer · 2015-06-22 02:58:03Z

10

valueOf may help you with whatever it is you are trying to do:

class Foo {
  constructor(value) {        this.value = value; }
  valueOf()          { return this.value;         }
  ^^^^^^^^^
}    

var foo = new Foo(4);
console.log(+foo) // 4

You have to make sure you understand how valueOf works. According to the docs:

JavaScript calls the valueOf method to convert an object to a primitive value.

So simply referring to the object will not invoke valueOf, of course; that would leave you with no way to reference the underlying object. It is used in situations where it is necessary to coerce the object into a primitive, such as in +foo.

valueOf has nothing to do with ES6 per se; it is available for all objects.

edited Jun 22, 2015 at 2:58

answered Jun 22, 2015 at 2:51

user663031

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