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).
In this typescript book section the author explains lazy loading with the following example:
import foo = require('foo');
export function loadFoo() {
// This is lazy loading `foo` and using the original module *only* as a type annotation
var _foo: typeof foo = require('foo');
// Now use `_foo` as a variable instead of `foo`.
}
According to the author typescript only loads the type of foo on the first call to require but on the second call when a foo var is created it loads the entire module required to create the var _foo.
How does this work. Can someone show a more detailed example of what's going on under the hood?
Typescript 2.4 now supports Dynamic Import Expressions where you can lazily import modules.
Here is the example:
async function getZipFile(name: string, files: File[]): Promise<File> {
const zipUtil = await import('./utils/create-zip-file');
const zipContents = await zipUtil.getContentAsBlob(files);
return new File(zipContents, name);
}
Behind the hood it is still using require as you can see in the transpiled code here
It's mentioned in the typescript handbook
The compiler detects whether each module is used in the emitted
JavaScript. If a module identifier is only ever used as part of a type
annotations and never as an expression, then no require call is
emitted for that module.
In this example, the first foo (the one without an underscore) is used only once in the type annotation as an argument of typeof, so the first require('foo') is omitted from the generated javascript code. You can check generated .js file to see that, and there would be only one call to the require at runtine, the 'second' one.
When loadFoo() is called, require('foo') is executed, calling built-in node.js require() function which loads foo module at runtime in the usual way.
I have AngularJS controller:
ArticleController.prototype = Object.create(BaseController.prototype);
/* #ngInject */
function ArticleController (CommunicationService){
//Some code not related with problem
}
Which is minified with gulp:
return gulp.src(pathsToMinify)
.pipe(require('gulp-ng-annotate')())
.pipe(require('gulp-uglify')())
.pipe(require('gulp-concat')('application.min.js'))
.pipe(gulp.dest('dist'));
And then I decided to migrate from plain Javascript to Typescript, starting with BaseController:
class BaseController {
constructor() {
//Some code not related with problem
}
}
After minification and concatenation, I got:
Uncaught TypeError: Cannot read property 'prototype' of undefined
Related to line:
ArticleController.prototype = Object.create(BaseController.prototype);
Then I realised that Typescript compiler otputs BaseController as variable with IIFE:
var BaseController = (function () {
function BaseController() {
}
BaseController.prototype.setPath = function (path) {
this._path = path;
};
//Some code not related with problem
return BaseController;
})();
Problem IMO is related with variable/function hoisting in Javascript - when I manually replace variable and IIFE with function:
function BaseController() {
}
//Some code not related with problem
It works properly. Is there any idea to dispose of this problem, like forcing Typescript compiler to output function instead of variable with IIFE? Or, I can not change it, and I have to deal with it in other way? Thank you in advance for any help, I am pretty new to Typescript and I didn't realize that I can find problems like this.
Likely, BaseController is not visible from ArticleController (because it is concatenated after for instance).
To avoid these sort of issues, write modular Typescript (es6 and/or commonjs style) and use webpack or browserify
I use Mocha/Chai for testing JavaScript front-end code and now we switched to TypeScript. I have several functions I want to test. But they shouldn't be exportable. Can I get an access to this functions and test it without adding export to them?
There is no way to access a non-exported module function.
module MyModule {
function privateFunction() {
alert("privateFunction");
}
}
MyModule.privateFunction(); // Generates a compiler error
However, leaving aside the question of validity of private method testing, here is what you can do.
Group your functions into an utility class, and then leverage the fact that private class members can be accessed via square bracket notation.
module MyModule {
export class UtilityClass {
private privateFunction() {
alert("privateFunction");
}
}
}
var utility = new MyModule.UtilityClass();
//utility.privateFunction(); Generates a compiler error
utility["privateFunction"](); // Alerts "privateFunction"
While it is not possible to access non-exported function directly there is still a way to export them in a "semi-hidden" way. One possible approach would be:
// In your library module declare internal functions as non-exported like normal.
function someInternalFunctionA(x: number): number {
return x;
}
function someInternalFunctionB(x: number): number {
return x;
}
// At the bottom, offer a public escape hatch for accessing certain functions
// you would like to be available for testing.
export const _private = {
someInternalFunctionA,
someInternalFunctionB,
};
On test side you can do:
import { _private } from "./myModule";
test("someInternalFunctionA", () => {
expect(_private.someInternalFunctionA(42)).toEqual(42);
});
What I like about the approach:
No need to mark someInternalFunctionA with export directly.
It should still be very obvious that the stuff under _private isn't officially part of the public interface.
As you can see in related questions the issue of testing private functions inside classes or modules is heavily debated on StackOverflow - The following might be an architectural solution to not even have that discussion:
If these functions feel important enough to be tested separately, but should not be accessible as part of a module, should they maybe be placed in their own module?
This would solve your issue of accessibility - they now are public functions in one module, and you can easily use them from within another module and not expose them as part of that module.
Best solution I found is to export the private function under different name so this name will remind you not to use this function anywhere else.
export const getPrice__test = getPrice;
function getPrice(): number {
return 10 + Math.Random() * 50;
}
But they shouldn't be exportable. Can I get an access to this functions and test it without adding export to them?
In general, no. It's possible to access private class members, but not unexported members of modules.
I would echo #Katana314's comments -- unit tests should not be concerning themselves with non-public methods. Trying to do so is an indication that you're testing the implementation details of the module rather than the contract the module claims to implement.
This is a total hack, but hey...
window.testing = {};
Then in your module:
module.exports = {
myPublicFunction: myPublicFunction
};
window.testing.myModule = {
myPublicFunction: myPublicFunction,
myPrivateFunction: myPrivateFunction
};
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;