Purpose of Static Methods in ECMAScript 6 Classes

问题:

What sort of issues with ES5 are static class methods in ES6 supposed to deal with?

The Babel documentation has the following example in its section regarding ES6 classes, though it does not actually state what this pattern accomplishes.

Classes support prototype-based inheritance, super calls, instance and static methods and constructors

class SkinnedMesh extends THREE.Mesh {
  constructor(geometry, materials) {
    super(geometry, materials);

    this.idMatrix = SkinnedMesh.defaultMatrix();
    this.bones = [];
    this.boneMatrices = [];
    //...
  }
  update(camera) {
    //...
    super.update();
  }
  static defaultMatrix() {
    return new THREE.Matrix4();
  }
}

回答1:

If you compile ES6 code with Babel, and some class contains a static method, ES5-generated code will be just adding that static function to the constructor function.

So, this ES6 ES2015 code:

class A {
   static doStuff() {}
}

...equals (in ES5):

function A() { }
A.doStuff = function() { };

Why you need static functions? Well, transpiled code won't support statics at all, since even functions are objects and static functions are turned into own properties of constructor function.

Static functions or properties can be used to implement factory pattern:

class A {
   static create() {
      // Specific factory method code
   } 
}

var instance = A.create();

Anyway, static member usage is a very diffuse topic and it goes out of scope of an objective answer. It has a lot of use cases and these are universal to any programming language.

回答2:

Consider class that contains only static methods:

class MyNamespace {
  static foo() { ... }
  static bar() { foo(); }
}

It is quite convenient way of organizing code - put stuff in namespaces.

MyNamespace.foo();
MyNamespace.bar();

That's other than standard static method use cases in other OOP languages.