ES6 - declare a prototype method on a class with an import statement

Yes. The class syntax is just (very sophisticated) syntactic sugar for constructor functions. So Car will still be a function with a prototype property and you can do exactly the same:

import getColor from './getColor';
// ...
Car.prototype.getColor = getColor;

However, that makes the method enumerable, as opposed to the methods created from the class syntax. So you might want to use Object.defineProperty instead.

You can still attach a method on a class' prototype; after-all, classes are just syntactic sugar over a "functional object", which is the old way of using a function to construct objects.

Since you want to use ES6, I'll use an ES6 import.

Minimal effort, using the prototype:

import getColor from 'path/to/module';

class Car {
    ...
}

Car.prototype.getColor = getColor;

As you can see, you still use the prototype property to attach a method, should you choose to.

---

Calling the module within a class' method:

Alternatively, if you don't want to use the prototype property, you can always have your method return the function from the module:

import getColor from 'path/to/module';

class Car {
    getColor () {
        return getColor.call(this);
    }
}

---

Using a Getter

You could also be a bit tricky and use a "getter" to achieve this in a different manner.

import getColor from 'path/to/module';

class Car {
    get getColor () { return getColor.bind(this) }
}

You could then use it simply by calling, myInstanceOfCar.getColor()

Or in a more semantic usage of a getter:

class Car {
    get color () { return getColor.call(this) }
}

// ...

const color = myInstanceOfCar.color;

Keep in mind that getters/setters cannot have the same name as properties that you set in the constructor. You will end up exceeding the maximum call-stack with infinite recursion when you try to use the setter to set that same property. Example: set foo (value) { this.foo = value }

---

ES2016 Class Properties

If you're using Babel to transpile (and are using experimental proposals), and want to use some ES2016, you can use the following syntax (but keep in mind that this applies the method to the object directly, and does not set it on the prototype):

import getColor from 'path/to/module';

class Car {
    getColor = getColor;
}

---

Optional binding w/ class properties

If you use the shorthand syntax for setting a property, you won't have to bind the method (setting is as a property changes what "this" refers to, essentially automatically binding it), but you certainly can, should you choose to (like if you'd like to bind something else):

getColor = getColor.bind(this);