I am in the process of moving an old JS codebase to modern JS. First step though is to add some tests to the current codebase. The current code base is essentially a bunch of individual JS files each wrapped in an IIFE.
This in itself is a problem for tests because, unless something is exposed to the global object, you cannot reach into the IIFE. Some of the code I am refactoring to be simple JS object with properties, which is attached to a namespace(namespace below is just a placeholder name) on the global object, for example:
var namespace = window.namespace || {};
var paymentsHandlerUtils = {
getNewValue: function(selectedAmount) {
'use strict';
return selectedAmount < 1 || isNaN(selectedAmount)
? ''
: '$' + selectedAmount;
},
getSelectedAmount: function(value) {
'use strict';
return value % 1 === 0 ? parseInt(value) : parseFloat(value).toFixed(2);
}
};
namespace.paymentsHandlerUtils = paymentsHandlerUtils;
My question is, how would you go about testing this with Jest? I have tried requiring the above as follows:
const paymentsHandlerUtils = require('../js/components/payments/payments-handler-utils.js');
This runs, but the paymentsHandlerUtils object is just an empty {}. Not surprising, as nothing is being returned by simply executing the JS. However, window.namespace is also undefined. Seems like the code is not being executed in the context of jsDOM, so the global(s) is not created.
Is there a way to get this to work, or is this simply not a use case for Jest? Thanks in advance.
I don’t think there is a way to access module globals when imported as it goes against the principle of encapsulated modules in the first place.
An alternative which would require the least refactoring would be to add the following code to all your modules:
if (typeof exports === "object") {
module.exports = paymentsHandlerUtils;
}
namespace.paymentsHandlerUtils = paymentsHandlerUtils;
It is inspired by the old UMD (Universal Module Definition). The condition detects if you are running in a commonjs environment and exports your variable. You will then be able to require it in your tests:
const paymentsHandlerUtils = require('../js/components/payments/payments-handler-utils.js');
Otherwise you need to use another test suite that works in the browser, because Jest doesn’t.
Good luck to your migrations!
In fact this is now possible with rewire. Here's what you have to do:
install rewire (npm i rewire)
in your jest file,
const rewire = require("'rewire');
const paymentsHandlerUtilsRewire = rewire('../js/components/payments/payments-handler-utils.js');
const namespace = paymentsHandlerUtilsRewire.__get__('namespace');
do what you were planning to do with namespace and namespace.paymentsHandlerUtils
Read more in the rewire repo; there's also babel-plugin-rewire for ES6+ (see, for instance, here).
I can't seem to find a description of the way I should export global variable from ES6 module. Is there a resource where it's defined?
The only solution that seems to work is referencing a global object, like window:
window['v'] = 3;
But what if this scripts runs in Node.js? Then I don't have window; I have global. But this code is not good:
var g = window || global;
g['v'] = 3;
I understand the concept of modules and don't use globals in my applications. However, having global variables during debugging in the console may be beneficial, especially when using bundlers like Webpack instead of loaders like SystemJs where you can easily import a module in a console.
There are several ways to have global values available in your application.
Using ES6 modules, you can create a constant which you export from your module. You can then import this from any other module or component, like so:
/* Constants.js */
export default {
VALUE_1: 123,
VALUE_2: "abc"
};
/* OtherModule.js */
import Constants from '../Constants';
console.log(Constants.VALUE_1);
console.log(Constants.VALUE_2);
Alternatively, some JS bundling tools provide a way to pass values into your components at build time.
For example, if you're using Webpack, you can use DefinePlugin to configure a few constants available at compile time, like so:
/* Webpack configuration */
const webpack = require('webpack');
/* Webpack plugins definition */
new webpack.DefinePlugin({
'VALUE_1': 123,
'VALUE_2': 'abc'
});
/* SomeComponent.js */
if (VALUE_1 === 123) {
// do something
}
You can grab the global object with an indirect eval call.
// this weird syntax grabs the global object
const global = (0,eval)("this");
// (0,eval) === eval; but the first one is an indirect evaluation
// inside indirect evaluation of eval, "this" is global object
// this takes advantage of that fact to identify "global"
// then set whatever global values you need
global.VALUE_1 = 123;
global.VALUE_2 = "abc";
You'll have to take care with the way your modules are loaded to ensure proper ordering.
more info: (1, eval)('this') vs eval('this') in JavaScript?
You can use globalThis:
function test(h) {
globalThis.testVar = h
}
test("This is a global var")
console.log(testVar)
I am using Gulp, Babelify and Browserify to compile ES2015 into ES5.
Folder structure:
project
- js
- level.js
- main.js
If I have the below defined module:
// level.js
var level = [];
level[0] = {
name: 'level1',
grid: {
width: GRID_WIDTH,
height: GRID_HEIGHT
}
};
export default level;
And then I have my main JavaScript file:
// main.js
import level from './level.js';
var siteGame = (function() {
var GRID_WIDTH = 50,
GRID_HEIGHT = 50;
console.log(level);
}());
How can I make sure that GRID_WIDTH and GRID_HEIGHT are available inside level.js?
At the moment I get an error, advising that GRID_WIDTH is undefined (from level.js)
The whole point of modules is that each has their own scope. If you want to "inject" data into a module, either export a function from level.js that accepts the dimensions, constructs the array/object and returns it, or put the dimensions in their own module and import it in level.js
You defined those variables inside a function - they live and die inside the btackets of this function and you cannot access them anywhere outside them. If you want to use those variables globally you can either -
Write them on the global window object as done in es5 - but is not very recommended for various reasons
Create a module which exports an object with all the variables you want to be exposed in you're app that way you can import that module and get all the variables you need.
I really recommend the second option over the first one.
What is the purpose of Node.js module.exports and how do you use it?
I can't seem to find any information on this, but it appears to be a rather important part of Node.js as I often see it in source code.
According to the Node.js documentation:
module
A reference to the current
module. In particular module.exports
is the same as the exports object. See
src/node.js for more information.
But this doesn't really help.
What exactly does module.exports do, and what would a simple example be?
module.exports is the object that's actually returned as the result of a require call.
The exports variable is initially set to that same object (i.e. it's a shorthand "alias"), so in the module code you would usually write something like this:
let myFunc1 = function() { ... };
let myFunc2 = function() { ... };
exports.myFunc1 = myFunc1;
exports.myFunc2 = myFunc2;
to export (or "expose") the internally scoped functions myFunc1 and myFunc2.
And in the calling code you would use:
const m = require('./mymodule');
m.myFunc1();
where the last line shows how the result of require is (usually) just a plain object whose properties may be accessed.
NB: if you overwrite exports then it will no longer refer to module.exports. So if you wish to assign a new object (or a function reference) to exports then you should also assign that new object to module.exports
It's worth noting that the name added to the exports object does not have to be the same as the module's internally scoped name for the value that you're adding, so you could have:
let myVeryLongInternalName = function() { ... };
exports.shortName = myVeryLongInternalName;
// add other objects, functions, as required
followed by:
const m = require('./mymodule');
m.shortName(); // invokes module.myVeryLongInternalName
This has already been answered but I wanted to add some clarification...
You can use both exports and module.exports to import code into your application like this:
var mycode = require('./path/to/mycode');
The basic use case you'll see (e.g. in ExpressJS example code) is that you set properties on the exports object in a .js file that you then import using require()
So in a simple counting example, you could have:
(counter.js):
var count = 1;
exports.increment = function() {
count++;
};
exports.getCount = function() {
return count;
};
... then in your application (web.js, or really any other .js file):
var counting = require('./counter.js');
console.log(counting.getCount()); // 1
counting.increment();
console.log(counting.getCount()); // 2
In simple terms, you can think of required files as functions that return a single object, and you can add properties (strings, numbers, arrays, functions, anything) to the object that's returned by setting them on exports.
Sometimes you'll want the object returned from a require() call to be a function you can call, rather than just an object with properties. In that case you need to also set module.exports, like this:
(sayhello.js):
module.exports = exports = function() {
console.log("Hello World!");
};
(app.js):
var sayHello = require('./sayhello.js');
sayHello(); // "Hello World!"
The difference between exports and module.exports is explained better in this answer here.
Note that the NodeJS module mechanism is based on CommonJS modules which are supported in many other implementations like RequireJS, but also SproutCore, CouchDB, Wakanda, OrientDB, ArangoDB, RingoJS, TeaJS, SilkJS, curl.js, or even Adobe Photoshop (via PSLib).
You can find the full list of known implementations here.
Unless your module use node specific features or module, I highly encourage you then using exports instead of module.exports which is not part of the CommonJS standard, and then mostly not supported by other implementations.
Another NodeJS specific feature is when you assign a reference to a new object to exports instead of just adding properties and methods to it like in the last example provided by Jed Watson in this thread. I would personally discourage this practice as this breaks the circular reference support of the CommonJS modules mechanism. It is then not supported by all implementations and Jed example should then be written this way (or a similar one) to provide a more universal module:
(sayhello.js):
exports.run = function() {
console.log("Hello World!");
}
(app.js):
var sayHello = require('./sayhello');
sayHello.run(); // "Hello World!"
Or using ES6 features
(sayhello.js):
Object.assign(exports, {
// Put all your public API here
sayhello() {
console.log("Hello World!");
}
});
(app.js):
const { sayHello } = require('./sayhello');
sayHello(); // "Hello World!"
PS: It looks like Appcelerator also implements CommonJS modules, but without the circular reference support (see: Appcelerator and CommonJS modules (caching and circular references))
Some few things you must take care if you assign a reference to a new object to exports and /or modules.exports:
1. All properties/methods previously attached to the original exports or module.exports are of course lost because the exported object will now reference another new one
This one is obvious, but if you add an exported method at the beginning of an existing module, be sure the native exported object is not referencing another object at the end
exports.method1 = function () {}; // exposed to the original exported object
exports.method2 = function () {}; // exposed to the original exported object
module.exports.method3 = function () {}; // exposed with method1 & method2
var otherAPI = {
// some properties and/or methods
}
exports = otherAPI; // replace the original API (works also with module.exports)
2. In case one of exports or module.exports reference a new value, they don't reference to the same object any more
exports = function AConstructor() {}; // override the original exported object
exports.method2 = function () {}; // exposed to the new exported object
// method added to the original exports object which not exposed any more
module.exports.method3 = function () {};
3. Tricky consequence. If you change the reference to both exports and module.exports, hard to say which API is exposed (it looks like module.exports wins)
// override the original exported object
module.exports = function AConstructor() {};
// try to override the original exported object
// but module.exports will be exposed instead
exports = function AnotherConstructor() {};
the module.exports property or the exports object allows a module to select what should be shared with the application
I have a video on module_export available here
When dividing your program code over multiple files, module.exports is used to publish variables and functions to the consumer of a module. The require() call in your source file is replaced with corresponding module.exports loaded from the module.
Remember when writing modules
Module loads are cached, only initial call evaluates JavaScript.
It's possible to use local variables and functions inside a module, not everything needs to be exported.
The module.exports object is also available as exports shorthand. But when returning a sole function, always use module.exports.
According to: "Modules Part 2 - Writing modules".
the refer link is like this:
exports = module.exports = function(){
//....
}
the properties of exports or module.exports ,such as functions or variables , will be exposed outside
there is something you must pay more attention : don't override exports .
why ?
because exports just the reference of module.exports , you can add the properties onto the exports ,but if you override the exports , the reference link will be broken .
good example :
exports.name = 'william';
exports.getName = function(){
console.log(this.name);
}
bad example :
exports = 'william';
exports = function(){
//...
}
If you just want to exposed only one function or variable , like this:
// test.js
var name = 'william';
module.exports = function(){
console.log(name);
}
// index.js
var test = require('./test');
test();
this module only exposed one function and the property of name is private for the outside .
There are some default or existing modules in node.js when you download and install node.js like http, sys etc.
Since they are already in node.js, when we want to use these modules we basically do like import modules, but why? because they are already present in the node.js. Importing is like taking them from node.js and putting them into your program. And then using them.
Whereas Exports is exactly the opposite, you are creating the module you want, let's say the module addition.js and putting that module into the node.js, you do it by exporting it.
Before I write anything here, remember, module.exports.additionTwo is same as exports.additionTwo
Huh, so that's the reason, we do like
exports.additionTwo = function(x)
{return x+2;};
Be careful with the path
Lets say you have created an addition.js module,
exports.additionTwo = function(x){
return x + 2;
};
When you run this on your NODE.JS command prompt:
node
var run = require('addition.js');
This will error out saying
Error: Cannot find module addition.js
This is because the node.js process is unable the addition.js since we didn't mention the path. So, we have can set the path by using NODE_PATH
set NODE_PATH = path/to/your/additon.js
Now, this should run successfully without any errors!!
One more thing, you can also run the addition.js file by not setting the NODE_PATH, back to your nodejs command prompt:
node
var run = require('./addition.js');
Since we are providing the path here by saying it's in the current directory ./ this should also run successfully.
A module encapsulates related code into a single unit of code. When creating a module, this can be interpreted as moving all related functions into a file.
Suppose there is a file Hello.js which include two functions
sayHelloInEnglish = function() {
return "Hello";
};
sayHelloInSpanish = function() {
return "Hola";
};
We write a function only when utility of the code is more than one call.
Suppose we want to increase utility of the function to a different file say World.js,in this case exporting a file comes into picture which can be obtained by module.exports.
You can just export both the function by the code given below
var anyVariable={
sayHelloInEnglish = function() {
return "Hello";
};
sayHelloInSpanish = function() {
return "Hola";
};
}
module.export=anyVariable;
Now you just need to require the file name into World.js inorder to use those functions
var world= require("./hello.js");
The intent is:
Modular programming is a software design technique that emphasizes
separating the functionality of a program into independent,
interchangeable modules, such that each contains everything necessary
to execute only one aspect of the desired functionality.
Wikipedia
I imagine it becomes difficult to write a large programs without modular / reusable code. In nodejs we can create modular programs utilising module.exports defining what we expose and compose our program with require.
Try this example:
fileLog.js
function log(string) { require('fs').appendFileSync('log.txt',string); }
module.exports = log;
stdoutLog.js
function log(string) { console.log(string); }
module.exports = log;
program.js
const log = require('./stdoutLog.js')
log('hello world!');
execute
$ node program.js
hello world!
Now try swapping ./stdoutLog.js for ./fileLog.js.
What is the purpose of a module system?
It accomplishes the following things:
Keeps our files from bloating to really big sizes. Having files with e.g. 5000 lines of code in it are usually real hard to deal with during development.
Enforces separation of concerns. Having our code split up into multiple files allows us to have appropriate file names for every file. This way we can easily identify what every module does and where to find it (assuming we made a logical directory structure which is still your responsibility).
Having modules makes it easier to find certain parts of code which makes our code more maintainable.
How does it work?
NodejS uses the CommomJS module system which works in the following manner:
If a file wants to export something it has to declare it using module.export syntax
If a file wants to import something it has to declare it using require('file') syntax
Example:
test1.js
const test2 = require('./test2'); // returns the module.exports object of a file
test2.Func1(); // logs func1
test2.Func2(); // logs func2
test2.js
module.exports.Func1 = () => {console.log('func1')};
exports.Func2 = () => {console.log('func2')};
Other useful things to know:
Modules are getting cached. When you are loading the same module in 2 different files the module only has to be loaded once. The second time a require() is called on the same module the is pulled from the cache.
Modules are loaded in synchronous. This behavior is required, if it was asynchronous we couldn't access the object retrieved from require() right away.
ECMAScript modules - 2022
From Node 14.0 ECMAScript modules are no longer experimental and you can use them instead of classic Node's CommonJS modules.
ECMAScript modules are the official standard format to package JavaScript code for reuse. Modules are defined using a variety of import and export statements.
You can define an ES module that exports a function:
// my-fun.mjs
function myFun(num) {
// do something
}
export { myFun };
Then, you can import the exported function from my-fun.mjs:
// app.mjs
import { myFun } from './my-fun.mjs';
myFun();
.mjs is the default extension for Node.js ECMAScript modules.
But you can configure the default modules extension to lookup when resolving modules using the package.json "type" field, or the --input-type flag in the CLI.
Recent versions of Node.js fully supports both ECMAScript and CommonJS modules. Moreover, it provides interoperability between them.
module.exports
ECMAScript and CommonJS modules have many differences but the most relevant difference - to this question - is that there are no more requires, no more exports, no more module.exports
In most cases, the ES module import can be used to load CommonJS modules.
If needed, a require function can be constructed within an ES module using module.createRequire().
ECMAScript modules releases history
Release
Changes
v15.3.0, v14.17.0, v12.22.0
Stabilized modules implementation
v14.13.0, v12.20.0
Support for detection of CommonJS named exports
v14.0.0, v13.14.0, v12.20.0
Remove experimental modules warning
v13.2.0, v12.17.0
Loading ECMAScript modules no longer requires a command-line flag
v12.0.0
Add support for ES modules using .js file extension via package.json "type" field
v8.5.0
Added initial ES modules implementation
You can find all the changelogs in Node.js repository
let test = function() {
return "Hello world"
};
exports.test = test;
Looking at a random source file of the express framework for NodeJS, there are two lines of the code that I do not understand (these lines of code are typical of almost all NodeJS files).
/**
* Expose `Router` constructor.
*/
exports = module.exports = Router;
and
/**
* Expose HTTP methods.
*/
var methods = exports.methods = require('./methods');
I understand that the first piece of code allows the rest of the functions in the file to be exposed to the NodeJS app, but I don't understand exactly how it works, or what the code in the line means.
What do exports and module.exports actually mean?
I believe the 2nd piece of code allows the functions in the file to access methods, but again, how exactly does it do this.
Basically, what are these magic words: module and exports?
To be more specific:
module is the global scope variable inside a file.
So if you call require("foo") then :
// foo.js
console.log(this === module); // true
It acts in the same way that window acts in the browser.
There is also another global object called global which you can write and read from in any file you want, but that involves mutating global scope and this is EVIL
exports is a variable that lives on module.exports. It's basically what you export when a file is required.
// foo.js
module.exports = 42;
// main.js
console.log(require("foo") === 42); // true
There is a minor problem with exports on it's own. The _global scope context+ and module are not the same. (In the browser the global scope context and window are the same).
// foo.js
var exports = {}; // creates a new local variable called exports, and conflicts with
// living on module.exports
exports = {}; // does the same as above
module.exports = {}; // just works because its the "correct" exports
// bar.js
exports.foo = 42; // this does not create a new exports variable so it just works
Read more about exports
To expand on Raynos's answer...
exports is basically an alias for module.exports - I recommend just not using it. You can expose methods and properties from a module by setting them on module.exports, as follows:
//file 'module1.js'
module.exports.foo = function () { return 'bar' }
module.exports.baz = 5
Then you get access to it in your code:
var module1 = require('module1')
console.log(module1.foo())
console.log(module1.baz)
You can also override module.exports entirely to simply provide a single object upon require:
//glorp.js
module.exports = function () {
this.foo = function () { return 'bar' }
this.baz = 5
return this // need to return `this` object here
}
Now you've got a nice prototype:
var g1 = new require('glorp')()
console.log(g1.foo())
console.log(g1.baz)
There are myriad other ways to play with module.exports and require. Just remember, require('foo') always returns the same instance even if you call it multiple times.
Note
For the following to work,
var g1 = new require('glorp')()
console.log(g1.foo())
console.log(g1.baz)
this has to be returned in the function that is assigned to module.exports. Otherwise, you'll get a TypeError:
console.log(g1.foo())
^
TypeError: Cannot read property 'foo' of undefined
You can find the best answer in node.js source code.
If someone is requiring your js module,
your script turns into a function by node as follows (see src/node.js).
// require function does this..
(function (exports, require, module, __filename, __dirname) {
... your javascript contents...
});
Node will wrap your script. Then above script will be executed as follows:
//module.js
var args = [self.exports, require, self, filename, dirname];
return compiledWrapper.apply(self.exports, args);
So in your script,
exports is just module.exports.
In your script, you can add something to this exports object (functions..).
require function will return this object. This is node.js's module system (commonJS specification).
But be careful not to modify module.exports. Otherwise your current exports will be meaningless.
module is an object that represents what that particular source file would like to publicly expose. Instead of having something akin to header files in the c/c++ world, you describe what the module exports by defining this object. the node runtime then uses this object to determine what about your module is 'public.'
its a similar concept to exporting functions from a dll in the compiled world. you have to define explicitly what functions can be accessed by the outside world. this helps with encapsulation and lets your organize your libraries in a clean way.
Module's code is wrapped in module.exports (The module, maybe composed by other module).
There are many ways to build a module, but this is one very common (and my personal favorite).
// Dependencies
// const module = require('module');
// Module object
var foo = {}
// Internal property
foo._a = 'a';
// "Public" property
foo.b = 'b';
// Method
foo.fu = function() { return 'fu' };
// Export
module.exports = foo;