CODEWITHSUNDEEP
javascripttypescriptecmascript-6importmodule
Dylan Kerler
Dylan Kerler

Reputation: 2187

How do I export a single instance of a class and import that same instance in multiple files?

I have a base file that contains a class:

File-1:

class Foo {
   constructor() {
     this.count = 0;
   }
}

export = new Foo();

Then I import this instance in two other files:

File-2:

import Foo = require("./File-1");
Foo.count = 6;
export const logFile2Count = () => console.log(Foo.count); // 6

File-3:

import Foo = require("./File-1");
export const logFile3Count = () => console.log(Foo.count); // 0

File-4:

import { logFile2Count } from "./File-2";
import { logFile3Count } from "./File-3";

logFile2Count(); // "6"
logFile3Count(); // "0" (Expected 6)

I want the state of Foo to stay persistent throughout all files that it's imported in. I.e. I want to import a reference of Foo in each file as oppose to a copy of Foo in each file. How can I do this?

Upvotes: 1

Views: 3344

Answers (3)

penguintheorem
penguintheorem

Reputation: 321

well, if there isn't any form of sync. between File-2.js and File-3.js modules, before reading from File-1.js (like File-2.js depends on File-3.js or viceversa) i think you can only set the value directly in the common dependency File-1.js to be 100% sure that the dependant modules read the same values.

This because the code becomes a kind of race condition

Upvotes: 0

Dylan Kerler
Dylan Kerler

Reputation: 2187

I am not 100% sure but after some experimentation this is because of the way require behaves in relation to the import statement (probably something to do with the way tsc/babel is transpiling the code); It runs the entire code in the file and then gives you the exported values. If you change to use an import statement then it will work because it gives you a reference to the class when exporting as oppose to running the whole file and giving you a new instance.

This will give you the behaviour you intend:

File-1:

class Foo {
   constructor() {
     this.count = 0;
   }
}

const foo = new Foo();
export foo;

File-2:

import { foo } from "./File-1";
/** -- snip -- **/

Upvotes: 1

Karthik Radhakrishnan
Karthik Radhakrishnan

Reputation: 944

Actually I tried the below in node js using require instead import. It seems to work as expected File-1.js

class Foo {
   constructor() {
     this.count = 0;
   }
}

module.exports = new Foo();

File-2.js

let Foo = require("./File-1");
Foo.count = 6;
module.exports.logFile2Count = () => console.log(Foo.count); // 6

File-3.js

let Foo = require("./File-1")
module.exports.logFile3Count = () => console.log(Foo.count); // 0

File-4.js

let  {logFile2Count} = require("./File-2");
let  {logFile3Count} = require("./File-3");

logFile2Count(); // "6"
logFile3Count(); // "0" (Expected 6)

This is the output I got

node File-4.js 
6
6

Upvotes: 1

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