无法在ES6类上调用静态函数

Question

I have a file called helpers.js in the 'helpers' folder. The contents are like below:

class Helpers {
    constructor(config) {
        if (this._singleton) {
            throw new Error('A singleton has already been created.');
        }

        this._singleton = this;
    }

    /**
     * Gets the singleton object.
     * @returns {Helpers}
     */
    static getSingleton() {
        return this._singleton;
    }
}

module.exports = Helpers;

Then in /helpers/user.js I want to get the helper's singleton instance. This is my code:

const helpers  = require('../helpers').getSingleton();

or

const Helpers  = require('../helpers');
const helpers  = Helpers.getSingleton();

The error I keep getting is:

TypeError: require(...).getSingleton is not a function

or

TypeError: Helpers.getSingleton is not a function

If I hover over Helpers in VSCode, I get this tooltip

And, whenever I hover over getSingleton() I get this tooltip:

So the path is correct, but it still gives me the errors.

Answer 1

The easiest way to implement the singleton pattern in JavaScript is to just not export the class at all, eg

class Helpers {}

let helper;
module.exports = function() {
   if (!helper) helpers = new Helpers();
   return helper;
};

// loaded with
var helpers = require('../helpers')(); // note the extra () to call it

or even better, since we aren't restricted to Java-like behavior, just skip the function entirely and do

class Helpers {}
module.exports = new Helpers();

// loaded with
var helpers = require('../helpers');

but then if all your module is exporting is a single instance of a class, there's very little reason to use a class in the first place. You might as well do

exports.helperMethodOne = function(){};
exports.helperMethodTwo = function(){};
exports.helperMethodThree = function(){};

// loaded with
var helpers = require('../helpers');

or

module.exports = {
  helperMethodOne() {},
  helperMethodTwo() {},
  helperMethodThree() {},
};

// loaded with
var helpers = require('../helpers');

Answer 2

Your require statement is wrong, but its hard to tell you precisely the right syntax without knowing your environment.

const config = require('/path/to/file');

Is typical. So try:

const Helpers = require('../helpers');

You wrote '../helpers.js' in your screenshot, not '../helpers'

You get the error:

TypeError: require(...).getSingleton is not a function

Because require(...) resolves to something else, like null , and null.getSingleton is not a function.

---

Also, you cannot reference this meaninfully inside a static context. this ought to only be used for class instances, not static members.

Answer 3

You can do something like this to use it as Singleton.getInstance() ;

class Singleton {
    static instance = new Singleton();
    static getInstance = () => Singleton.instance;

    constructor() {
        throw new Error('Use Singleton.getInstance()');
    }
}

module.exports = Singleton;

or even something more sneaky and use it as new Singleton()

class Singleton {
    static instance;
    constructor() {
        if (!Singleton.instance) {
            Singleton.instance = this;
        }
        return Singleton.instance;
    }
}

module.exports = Singleton;