Javascript对象函数返回null

Question

I'm trying to do a very simple OOP in Javascript (Node.js) but having issues. I tried everything already, including searching, but did not find an answer.

Basically, I have this file Test.js:

class Test {

constructor(){
    this.name = 'Hey';
    this.config = 'null!';
    console.log('this.config: ' + this.config);
}

config(msg){
    this.config = msg;
    console.log('new this.config: ' + this.config);
}

}

module.exports = Test;

(I also tried this:)

function Test()
{
    this.name = 'Hey';
    this.config = 'null!';
    console.log('this.config: ' + this.config);
}

Test.config = function(msg) // and Test.prototype.config
{
    this.config = msg;
    console.log('new this.config: ' + this.config);
}

module.exports = Test;

And I have this other file app.js:

var TestModule = require('./Test.js');
var Test = new TestModule();
var test = Test.config('hi');

Other way I've tried:

var TestModule = require('./Test.js');
var Test = new TestModule().config('hi');

and also did not work.

I tried many different things already, but no matter what, when I try to run the config function in the same instance, the object turns null... does anyone know why that happens? Maybe I'm missing something really obvious.

Answer 1

You are assigning your var Test to be the return value of the config function.

var test = Test.config('hi!');

As config does not return anything, this will result in test being null.

You should either make your config method return something (this would be a form of the "method chaining" design pattern ), or simply not assign the result of the config call to a variable.

For example, you could simply do this:

var test = new TestModule();
test.config('hi!');
// the 'test' variable still contains a reference to your test module

Answer 2

You first snippet is correct

class Test {

    constructor() {
      this.name = 'Hey';
      this.config = 'null!';
      console.log('this.config: ' + this.config);
    }

    config(msg) {
      this.config = msg;
      console.log('new this.config: ' + this.config);
    }

  }

  module.exports = Test;

config is an instance method not a class method or static method.

You need to call config() on a Test instance. like

var Test = require('./Test.js');
var testObj = new Test();

Now testObj is instance and you can call config() method on this object.

test.config('Hi');

It will print/log a message but it will not return any thing but undefined because you are not returning anything from that method.

I hope this explains the problem.