您如何在自定义类上调用静态方法？ JS

Question

In javascript, I've created a class Group that creates an empty array named items and returns it.

But I'm trying to create a static method from(){} that takes an iterable object and creates a new Group from that iterable obj.

I'm trying this.items.push() but its not working how I'd expect it to

class Group(){
 constructor(){
   this.items = []
 }
 from(obj){
  for(let item of obj){this.items.push(item)}
 }
}

i'd implement it like so:

let group = Group.from([10, 11))//random numbers     

it unfortunately returns the following error:

TypeError: Cannot read property 'push' of undefined (line 37 in function 
Function.from)

Answer 1

You have a syntax error:

class Group(){

should be:

class Group { // no parameter list here

as in the class syntax, the parameters are defined in the constructor function.

Where a constructor adds methods to "class" instances as below, the method is available to instances, not the constructor itself. You you have to create an instance, then call the method:

 class Group { constructor () { this.items = []; } from (obj) { for (let item of obj) { this.items.push(item); } } } var group = new Group(); group.from([1,2]); console.log(group.items); // 1, 2 console.log(typeof Group.from); // undefined 

Answer 2

Although there are drafts for new static syntax on JS classes, you'll currently need to use the prototype for static methods / properties.

class Group {
  constructor(){
    this.items = []
  }
}

Group.prototype.from = function(obj){
  var group = new Group;
  for(let item of obj) group.items.push(item);
  return group;
}

let group = aGroudInstance.from([10, 11]);

You probably want to simply add the method onto the Group class (object) like so:

Group.from = function(...) ...;

As it's not actually making use of being a static method, and would unnecesarily require an instance of a Group -- unless you used it like so: Group.prototype.from(...) .

It's a factory function.