I'm trying to load a JS file from a bookmarklet. The JS file has this JS that wraps the module:
(function (root, factory) {
if (typeof module === 'object' && module.exports) {
// Node/CommonJS
module.exports = factory();
} else if (typeof define === 'function' && define.amd) {
// AMD. Register as an anonymous module.
define(factory);
} else {
// Browser globals
root.moduleGlobal = factory();
}
}(this, function factory() {
// module script is in here
return moduleGlobal;
}));
Because of this, if the webpage uses RequireJS, the script will not export a global when it loads. To get around this I temporarily set define to null, load the script, then reset define to its original value:
function loadScript(url, cb) {
var s = document.createElement('script');
s.src = url;
s.defer = true;
var avoidRequireJS = typeof define === 'function' && define.amd;
if (avoidRequireJS) {
var defineTmp = define;
define = null;
}
s.onload = function() {
if (avoidRequireJS) define = defineTmp;
cb();
};
document.body.appendChild(s);
}
This works, but it seems to me like it could be problematic to change a global variable when other parts of the application could depend on it. Is there a better way to go about this?
You may fetch the script using XMLHttpRequest, jQuery.ajax or the new Fetch API.
This will allow you to manipulate the script and reassign define before executing it. Two options:
Have the module export a global by wrapping the script with:
(function(define){ ... })(null);
Handle the module exports yourself by wrapping the script with:
(function(define, module){ ... })((function() {
function define(factory) {
var exports = factory();
}
define.amd = true;
return define;
})());
You can then load it using a new <script> element or eval 😲.
Note that when using XHR, you may have to address CORS issues.
If you can use the AJAX method above, that will be best. But as stated, you will need to deal with CORS issues, which is not always trivial - even impossible if you do not control the origin server.
Here is a technique which uses an iframe to load the script in an isolated context, allowing the script to export its global object. We then grab the global object and copy it to the parent. This technique does not suffer from CORS restrictions.
(fiddle: https://jsfiddle.net/qu0pxesd/)
function loadScript (url, exportName) {
var iframe = document.createElement('iframe');
Object.assign(iframe.style, {
position: 'fixed',
top: '-9999em',
width: '0px'
});
var script = document.createElement('script');
script.onload = function () {
window[exportName] = iframe.contentWindow[exportName];
document.body.removeChild(iframe);
}
script.src = url;
document.body.appendChild(iframe);
iframe.contentWindow.document.open();
iframe.contentWindow.document.appendChild(script);
iframe.contentWindow.document.close();
}
loadScript('https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js', 'jQuery');
I ran a quick test to see if a memory leak would happen from deleting the iframe, and it appears to be memory safe. Here's the snapshot of loading a script 100 times, resulting in 100 different iframes and 100 different instances of jQuery loading.
The parent window's jQuery variable is continuously overwritten, meaning only the last one prevails and all previous references are cleaned up. This is not entirely scientific and you will need to do your own testing, but this should be safe enough to get you started.
Update: The above code requires that you know the name of the exported object, which is not always known. Some modules may export multiple variables too. For example, jQuery exports both $ and jQuery. The following fiddle illustrates a technique for solving this issue by copying any global objects which did not exist before the script was loaded:
https://jsfiddle.net/qu0pxesd/3/
Which approach would work best really depends on the specific needs of the project. Context would determine which one I'd use.
Undefining define Temporarily
I'm mentioning it because you tried it.
DON'T DO THIS!
The approach of undefining define before you load your script and restoring it after is not safe. In the general case, it is possible for other code on the page to perform a require call that will resolve after you've undefined define and before you've defined it again. After you do document.body.appendChild(s); you're handing back control to the JavaScript engine, which is free to immediately execute scripts that were required earlier. If the scripts are AMD module, they'll either bomb or install themselves incorrectly.
Wrapping the Script
As Dheeraj V.S. suggests, you can wrap the script to make define locally undefined:
(function(define) { /* original module code */ }())
can work for trivial cases like the one you show in your question. However, cases where the script you try to load actually has dependencies on other libraries can cause issues when it comes to dealing with the dependencies. Some examples:
The page loads jQuery 2.x but the script you are trying to load depends on a feature added in jQuery 3.x. Or the page loads Lodash 2 but the script needs Lodash 4, or vice-versa. (There are huge differences between Lodash 2 and 4.)
The script needs a library that is not otherwise loaded by something else. So now you are responsible for producing the machinery that will load the library.
Using RequireJS Contexts
RequireJS is capable of isolating multiple configurations from one another by defining a new context. Your bookmarklet could define a new context that configures enough paths for the script you are trying to load to load itself and its dependencies:
var myRequire = require.config({
// Within the scope of the page, the context name must be unique to
// your bookmarklet.
context: "Web Designer's Awesome Bookmarklet",
paths: {
myScript: "https://...",
jquery: "https://code.jquery.com/jquery-3.2.1.min.js",
},
map: {...},
// Whatever else you may want.
});
myRequire(["myScript"]);
When you use contexts like this, you want to save the return value of require.config because it is a require call that uses your context.
Creating a Bundle with Webpack
(Or you could use Browserify or some other bundler. I'm more familiar with Webpack.)
You could use Webpack to consume all the AMD modules necessary for the script you are trying to load to produce a bundle that exports its "module" as a global. At a minimum, you'll need something like this in your configuration:
// Tell Webpack what module constitutes the entry into the bundle.
entry: "./MyScript.js",
output: {
// This is the name under which it will be available.
library: "MyLibrary",
// Tell Webpack to make it globally available.
libraryTarget: "global",
// The final bundle will be ./some_directory/MyLibrary.js
path: "./some_directory/",
filename: "MyLibrary.js",
}
Once this is done, the bookmarklet only needs to insert a new script element that points to the produced bundle and no longer has to worry about wrapping anything or dealing with dependencies.
If it were me, I would have the url provide the hint as to how to load the module. Instead of having just a "scripts/" directory -> I would make "scripts/amd/", "scripts/require/", etc. Then query the url for "amd", "require", etc. within your loadScript method... using, e.g.,
if (url.includes('amd')) {
// do something
} else if (url.includes('require')) {
// do something different
}
That should let you avoid the global var entirely. It might also provide a better structure for your app in general.
You could also return an object with a script property and loadType property that specifies amd, require, etc... but imho the first option would be the quickest and save you some additional typing.
Cheers
I'm trying to get JavaScript to read/write to a PostgreSQL database. I found this project on GitHub. I was able to get the following sample code to run in Node.
var pg = require('pg'); //native libpq bindings = `var pg = require('pg').native`
var conString = "tcp://postgres:1234#localhost/postgres";
var client = new pg.Client(conString);
client.connect();
//queries are queued and executed one after another once the connection becomes available
client.query("CREATE TEMP TABLE beatles(name varchar(10), height integer, birthday timestamptz)");
client.query("INSERT INTO beatles(name, height, birthday) values($1, $2, $3)", ['Ringo', 67, new Date(1945, 11, 2)]);
client.query("INSERT INTO beatles(name, height, birthday) values($1, $2, $3)", ['John', 68, new Date(1944, 10, 13)]);
//queries can be executed either via text/parameter values passed as individual arguments
//or by passing an options object containing text, (optional) parameter values, and (optional) query name
client.query({
name: 'insert beatle',
text: "INSERT INTO beatles(name, height, birthday) values($1, $2, $3)",
values: ['George', 70, new Date(1946, 02, 14)]
});
//subsequent queries with the same name will be executed without re-parsing the query plan by postgres
client.query({
name: 'insert beatle',
values: ['Paul', 63, new Date(1945, 04, 03)]
});
var query = client.query("SELECT * FROM beatles WHERE name = $1", ['John']);
//can stream row results back 1 at a time
query.on('row', function(row) {
console.log(row);
console.log("Beatle name: %s", row.name); //Beatle name: John
console.log("Beatle birth year: %d", row.birthday.getYear()); //dates are returned as javascript dates
console.log("Beatle height: %d' %d\"", Math.floor(row.height/12), row.height%12); //integers are returned as javascript ints
});
//fired after last row is emitted
query.on('end', function() {
client.end();
});
Next I tried to make it run on a webpage, but nothing seemed to happen. I checked on the JavaScript console and it just says "require not defined".
So what is this "require"? Why does it work in Node but not in a webpage?
Also, before I got it to work in Node, I had to do npm install pg. What's that about? I looked in the directory and didn't find a file pg. Where did it put it, and how does JavaScript find it?
So what is this "require?"
require() is not part of the standard JavaScript API. But in Node.js, it's a built-in function with a special purpose: to load modules.
Modules are a way to split an application into separate files instead of having all of your application in one file. This concept is also present in other languages with minor differences in syntax and behavior, like C's include, Python's import, and so on.
One big difference between Node.js modules and browser JavaScript is how one script's code is accessed from another script's code.
In browser JavaScript, scripts are added via the <script> element. When they execute, they all have direct access to the global scope, a "shared space" among all scripts. Any script can freely define/modify/remove/call anything on the global scope.
In Node.js, each module has its own scope. A module cannot directly access things defined in another module unless it chooses to expose them. To expose things from a module, they must be assigned to exports or module.exports. For a module to access another module's exports or module.exports, it must use require().
In your code, var pg = require('pg'); loads the pg module, a PostgreSQL client for Node.js. This allows your code to access functionality of the PostgreSQL client's APIs via the pg variable.
Why does it work in node but not in a webpage?
require(), module.exports and exports are APIs of a module system that is specific to Node.js. Browsers do not implement this module system.
Also, before I got it to work in node, I had to do npm install pg. What's that about?
NPM is a package repository service that hosts published JavaScript modules. npm install is a command that lets you download packages from their repository.
Where did it put it, and how does Javascript find it?
The npm cli puts all the downloaded modules in a node_modules directory where you ran npm install. Node.js has very detailed documentation on how modules find other modules which includes finding a node_modules directory.
Alright, so let's first start with making the distinction between Javascript in a web browser, and Javascript on a server (CommonJS and Node).
Javascript is a language traditionally confined to a web browser with a limited global context defined mostly by what came to be known as the Document Object Model (DOM) level 0 (the Netscape Navigator Javascript API).
Server-side Javascript eliminates that restriction and allows Javascript to call into various pieces of native code (like the Postgres library) and open sockets.
Now require() is a special function call defined as part of the CommonJS spec. In node, it resolves libraries and modules in the Node search path, now usually defined as node_modules in the same directory (or the directory of the invoked javascript file) or the system-wide search path.
To try to answer the rest of your question, we need to use a proxy between the code running in the the browser and the database server.
Since we are discussing Node and you are already familiar with how to run a query from there, it would make sense to use Node as that proxy.
As a simple example, we're going to make a URL that returns a few facts about a Beatle, given a name, as JSON.
/* your connection code */
var express = require('express');
var app = express.createServer();
app.get('/beatles/:name', function(req, res) {
var name = req.params.name || '';
name = name.replace(/[^a-zA_Z]/, '');
if (!name.length) {
res.send({});
} else {
var query = client.query('SELECT * FROM BEATLES WHERE name =\''+name+'\' LIMIT 1');
var data = {};
query.on('row', function(row) {
data = row;
res.send(data);
});
};
});
app.listen(80, '127.0.0.1');
I noticed that whilst the other answers explained what require is and that it is used to load modules in Node they did not give a full reply on how to load node modules when working in the Browser.
It is quite simple to do. Install your module using npm as you describe, and the module itself will be located in a folder usually called node_modules.
Now the simplest way to load it into your app is to reference it from your html with a script tag which points at this directory. i.e if your node_modules directory is in the root of the project at the same level as your index.html you would write this in your index.html:
<script src="node_modules/ng"></script>
That whole script will now be loaded into the page - so you can access its variables and methods directly.
There are other approaches which are more widely used in larger projects, such as a module loader like require.js. Of the two, I have not used Require myself, but I think it is considered by many people the way to go.
It's used to load modules. Let's use a simple example.
In file circle_object.js:
var Circle = function (radius) {
this.radius = radius
}
Circle.PI = 3.14
Circle.prototype = {
area: function () {
return Circle.PI * this.radius * this.radius;
}
}
We can use this via require, like:
node> require('circle_object')
{}
node> Circle
{ [Function] PI: 3.14 }
node> var c = new Circle(3)
{ radius: 3 }
node> c.area()
The require() method is used to load and cache JavaScript modules. So, if you want to load a local, relative JavaScript module into a Node.js application, you can simply use the require() method.
Example:
var yourModule = require( "your_module_name" ); //.js file extension is optional
Necromancing.
IMHO, the existing answers leave much to be desired.
At first, it's very confusing.
You have a (nowhere defined) function "require", which is used to get modules.
And in said (CommonJS) modules, you can use require, exports and module, WITHOUT THEM EVER BEING DEFINED.
Not that it would be new that you could use undefined variables in JS, but you couldn't use an undefined function.
So it looks a little like magic at first.
But all magic is based on deception.
When you dig a little deeper, it turns out it is really quite simple:
Require is simply a (non-standard) function defined at global scope.
(global scope = window-object in browser, global-object in NodeJS).
Note that by default, the "require function" is only implemented in NodeJS, not in the browser.
Also, note that to add to the confusion, for the browser, there is RequireJS, which, despite the name containing the characters "require", RequireJS absolutely does NOT implement require/CommonJS - instead RequireJS implements AMD, which is something similar, but not the same (aka incompatible).
That last one is just one important thing you have to realize on your way to understanding require.
Now, as such, to answer the question "what is require", we "simply" need to know what this function does.
This is perhaps best explained with code.
Here's a simple implementation by Michele Nasti, the code you can find on his github page.
Let's call our minimalisc implementation of the require function "myRequire":
function myRequire(name)
{
console.log(`Evaluating file ${name}`);
if (!(name in myRequire.cache)) {
console.log(`${name} is not in cache; reading from disk`);
let code = fs.readFileSync(name, 'utf8');
let module = { exports: {} };
myRequire.cache[name] = module;
let wrapper = Function("require, exports, module", code);
wrapper(myRequire, module.exports, module);
}
console.log(`${name} is in cache. Returning it...`);
return myRequire.cache[name].exports;
}
myRequire.cache = Object.create(null);
window.require = myRequire;
const stuff = window.require('./main.js');
console.log(stuff);
Now you notice, the object "fs" is used here.
For simplicity's sake, Michele just imported the NodeJS fs module:
const fs = require('fs');
Which wouldn't be necessary.
So in the browser, you could make a simple implementation of require with a SYNCHRONOUS XmlHttpRequest:
const fs = {
file: `
// module.exports = \"Hello World\";
module.exports = function(){ return 5*3;};
`
, getFile(fileName: string, encoding: string): string
{
// https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/Synchronous_and_Asynchronous_Requests
let client = new XMLHttpRequest();
// client.setRequestHeader("Content-Type", "text/plain;charset=UTF-8");
// open(method, url, async)
client.open("GET", fileName, false);
client.send();
if (client.status === 200)
return client.responseText;
return null;
}
, readFileSync: function (fileName: string, encoding: string): string
{
// this.getFile(fileName, encoding);
return this.file; // Example, getFile would fetch this file
}
};
Basically, what require thus does, is it downloads a JavaScript-file, evals it in an anonymous namespace (aka Function), with the parameters "require", "exports" and "module", and returns the exports, meaning an object's public functions and properties.
Note that this evaluation is recursive: you require files, which themselfs can require files.
This way, all "global" variables used in your module are variables in the require-wrapper-function namespace, and don't pollute the global scope with unwanted variables.
Also, this way, you can reuse code without depending on namespaces, so you get "modularity" in JavaScript. "modularity" in quotes, because this is not exactly true, though, because you can still write window.bla/global.bla, and hence still pollute the global scope... Also, this establishes a separation between private and public functions, the public functions being the exports.
Now instead of saying
module.exports = function(){ return 5*3;};
You can also say:
function privateSomething()
{
return 42:
}
function privateSomething2()
{
return 21:
}
module.exports = {
getRandomNumber: privateSomething
,getHalfRandomNumber: privateSomething2
};
and return an object.
Also, because your modules get evaluated in a function with parameters
"require", "exports" and "module", your modules can use the undeclared variables "require", "exports" and "module", which might be startling at first. The require parameter there is of course a pointer to the require function saved into a variable.
Cool, right ?
Seen this way, require looses its magic, and becomes simple.
Now, the real require-function will do a few more checks and quirks, of course, but this is the essence of what that boils down to.
Also, in 2020, you should use the ECMA implementations instead of require:
import defaultExport from "module-name";
import * as name from "module-name";
import { export1 } from "module-name";
import { export1 as alias1 } from "module-name";
import { export1 , export2 } from "module-name";
import { foo , bar } from "module-name/path/to/specific/un-exported/file";
import { export1 , export2 as alias2 , [...] } from "module-name";
import defaultExport, { export1 [ , [...] ] } from "module-name";
import defaultExport, * as name from "module-name";
import "module-name";
And if you need a dynamic non-static import (e.g. load a polyfill based on browser-type), there is the ECMA-import function/keyword:
var promise = import("module-name");
note that import is not synchronous like require.
Instead, import is a promise, so
var something = require("something");
becomes
var something = await import("something");
because import returns a promise (asynchronous).
So basically, unlike require, import replaces fs.readFileSync with fs.readFileAsync.
async readFileAsync(fileName, encoding)
{
const textDecoder = new TextDecoder(encoding);
// textDecoder.ignoreBOM = true;
const response = await fetch(fileName);
console.log(response.ok);
console.log(response.status);
console.log(response.statusText);
// let json = await response.json();
// let txt = await response.text();
// let blo:Blob = response.blob();
// let ab:ArrayBuffer = await response.arrayBuffer();
// let fd = await response.formData()
// Read file almost by line
// https://developer.mozilla.org/en-US/docs/Web/API/ReadableStreamDefaultReader/read#Example_2_-_handling_text_line_by_line
let buffer = await response.arrayBuffer();
let file = textDecoder.decode(buffer);
return file;
} // End Function readFileAsync
This of course requires the import-function to be async as well.
"use strict";
async function myRequireAsync(name) {
console.log(`Evaluating file ${name}`);
if (!(name in myRequireAsync.cache)) {
console.log(`${name} is not in cache; reading from disk`);
let code = await fs.readFileAsync(name, 'utf8');
let module = { exports: {} };
myRequireAsync.cache[name] = module;
let wrapper = Function("asyncRequire, exports, module", code);
await wrapper(myRequireAsync, module.exports, module);
}
console.log(`${name} is in cache. Returning it...`);
return myRequireAsync.cache[name].exports;
}
myRequireAsync.cache = Object.create(null);
window.asyncRequire = myRequireAsync;
async () => {
const asyncStuff = await window.asyncRequire('./main.js');
console.log(asyncStuff);
};
Even better, right ?
Well yea, except that there is no ECMA-way to dynamically import synchronously (without promise).
Now, to understand the repercussions, you absolutely might want to read up on promises/async-await here, if you don't know what that is.
But very simply put, if a function returns a promise, it can be "awaited":
"use strict";
function sleep(interval)
{
return new Promise(
function (resolve, reject)
{
let wait = setTimeout(function () {
clearTimeout(wait);
//reject(new Error(`Promise timed out ! (timeout = ${timeout})`));
resolve();
}, interval);
});
}
The promise would then normally be used like this:
function testSleep()
{
sleep(3000).then(function ()
{
console.log("Waited for 3 seconds");
});
}
But when you return a promise, you can also use await, which means we get rid of the callback (sort of - actually, it is being replaced with a state-machine in the compiler/interpreter).
This way, we make asynchronous code feel like synchronous, so we now can use try-catch for error-handling.
Note that if you want to use await in a function, that function must be declared async (hence async-await).
async function testSleep()
{
await sleep(5000);
console.log("i waited 5 seconds");
}
And also please note that in JavaScript, there is no way to call an async function (blockingly) from a synchronous one (the ones you know). So if you want to use await (aka ECMA-import), all your code needs to be async, which most likely is a problem, if everything isn't already async...
An example of where this simplified implementation of require fails, is when you require a file that is not valid JavaScript, e.g. when you require css, html, txt, svg and images or other binary files.
And it's easy to see why:
If you e.g. put HTML into a JavaScript function body, you of course rightfully get
SyntaxError: Unexpected token '<'
because of Function("bla", "<doctype...")
Now, if you wanted to extend this to for example include non-modules, you could just check the downloaded file-contents for code.indexOf("module.exports") == -1, and then e.g. eval("jquery content") instead of Func (which works fine as long as you're in the browser). Since downloads with Fetch/XmlHttpRequests are subject to the same-origin-policy, and integrity is ensured by SSL/TLS, the use of eval here is rather harmless, provided you checked the JS files before you added them to your site, but that much should be standard-operating-procedure.
Note that there are several implementations of require-like functionality:
the CommonJS (CJS) format, used in Node.js, uses a require function and module.exports to define dependencies and modules. The npm ecosystem is built upon this format. (this is what is implemented above)
the Asynchronous Module Definition (AMD) format, used in browsers, uses a define function to define modules. (basically, this is overcomplicated archaic crap that you wouldn't ever want to use). Also, AMD is the format that is implemented by RequireJS (note that despite the name containing the characters "require", AMD absolutely is NOT CommonJS).
the ES Module (ESM) format. As of ES6 (ES2015), JavaScript supports a native module format. It uses an export keyword to export a module’s public API and an import keyword to import it. This is the one you should use if you don't give a flying f*ck about archaic browsers, such as Safari and IE/EdgeHTML.
the System.register format, designed to support ES6 modules within ES5. (the one you should use, if you need support for older browsers (Safari & IE & old versions of Chrome on mobile phones/tablets), because it can load all formats [for some, plugins are required], can handle cyclic-dependencies, and CSS and HTML - don't define your modules as system.register, though - the format is rather complicated, and remember, it can read the other easier formats)
the Universal Module Definition (UMD) format, compatible to all the above mentioned formats (except ECMA), used both in the browser and in Node.js. It’s especially useful if you write modules that can be used in both NodeJS and the browser. It's somewhat flawed, as it doesn't support the latest ECMA modules, though (maybe this will get fixed) - use System.register instead.
Important sidenote on the function argument "exports":
JavaScript uses call-by-value-sharing - meaning objects are passed as a pointer, but the pointer-value itselfs is passed BY VALUE, not by reference. So you can't override exports by assigning it a new object. Instead, if you want to override exports, you need to assign the new object to module.exports - because hey, module is the pointer passed by value, but exports in module.exports is the reference to the original exports pointer.
Important sidenote on module-Scope:
Modules are evaluated ONCE, and then cached by require.
That means all your modules have a Singleton scope.
If you want a non-singleton scope, you have to do something like:
var x = require("foo.js").createInstance();
or simply
var x = require("foo.js")();
with appropriate code returned by your module.
If you need CommonJS-support for the browser (IE5+, Chrome, Firefox),
check out my code in my comment on Michele Nasti's project
You know how when you are running JavaScript in the browser, you have access to variables like "window" or Math? You do not have to declare these variables, they have been written for you to use whenever you want.
Well, when you are running a file in the Node.js environment, there is a variable that you can use. It is called "module" It is an object. It has a property called "exports." And it works like this:
In a file that we will name example.js, you write:
example.js
module.exports = "some code";
Now, you want this string "some code" in another file.
We will name the other file otherFile.js
In this file, you write:
otherFile.js
let str = require('./example.js')
That require() statement goes to the file that you put inside of it, finds whatever data is stored on the module.exports property. The let str = ... part of your code means that whatever that require statement returns is stored to the str variable.
So, in this example, the end-result is that in otherFile.js you now have this:
let string = "some code";
or -
let str = ('./example.js').module.exports
Note:
the file-name that is written inside of the require statement: If it is a local file, it should be the file-path to example.js. Also, the .js extension is added by default, so I didn't have to write it.
You do something similar when requiring node.js libraries, such as Express. In the express.js file, there is an object named 'module', with a property named 'exports'.
So, it looks something like along these lines, under the hood (I am somewhat of a beginner so some of these details might not be exact, but it's to show the concept:
express.js
module.exports = function() {
//It returns an object with all of the server methods
return {
listen: function(port){},
get: function(route, function(req, res){}){}
}
}
If you are requiring a module, it looks like this:
const moduleName = require("module-name");
If you are requiring a local file, it looks like this:
const localFile = require("./path/to/local-file");
(notice the ./ at the beginning of the file name)
Also note that by default, the export is an object .. eg module.exports = {} So, you can write module.exports.myfunction = () => {} before assigning a value to the module.exports. But you can also replace the object by writing module.exports = "I am not an object anymore."
Two flavours of module.exports / require:
(see here)
Flavour 1
export file (misc.js):
var x = 5;
var addX = function(value) {
return value + x;
};
module.exports.x = x;
module.exports.addX = addX;
other file:
var misc = require('./misc');
console.log("Adding %d to 10 gives us %d", misc.x, misc.addX(10));
Flavour 2
export file (user.js):
var User = function(name, email) {
this.name = name;
this.email = email;
};
module.exports = User;
other file:
var user = require('./user');
var u = new user();
we are using require.js in our project and we need to override the setTimeout in line 705 , this is the code which we need to ignore/omit somehow this setTimeout at all(I mean run over it) ,the problem that if I change it in the open source code explicit when I change version the code will be lost,How should I override this setTimout from outside only for the require.js file and keep it as long as I use this lib, is it possible to do it in elegant way in JS globally ?
https://github.com/jrburke/requirejs/blob/master/require.js
This is line 705
//If still waiting on loads, and the waiting load is something
//other than a plugin resource, or there are still outstanding
//scripts, then just try back later.
if ((!expired || usingPathFallback) && stillLoading) {
//Something is still waiting to load. Wait for it, but only
//if a timeout is not already in effect.
if ((isBrowser || isWebWorker) && !checkLoadedTimeoutId) {
checkLoadedTimeoutId = setTimeout(function () {
checkLoadedTimeoutId = 0;
checkLoaded();
}, 50);
}
}
FYI ,The reason that we do it is
Chrome: timeouts/interval suspended in background tabs?
You've stated your goal is to work around the throttling that Chrome performs on setTimeout for tabs that are in the background. I do not think it is a good idea to do so but if you must, then you should definitely patch RequireJS instead of messing with setTimeout globally. You said:
if I change it in the open source code explicit when I change version the code will be lost
This is true only if you do not use a sensible method to perform the change. It is possible to do it sensibly. For instance, you can use Gulp to take the require.js file installed in node_modules (after you install RequireJS with npm) and produce a patched file in build. Then you use this patched file in your application. Here is the gulpfile.js:
var gulp = require("gulp");
// Bluebird is a good implementation of promises.
var Promise = require("bluebird");
// fs-extra produces a `fs` module with additional functions like
// `ensureDirAsync` which is used below.
var fs = require("fs-extra");
// Make it so that for each the function in fs that is asynchronous
// and takes a callback (e.g. `fs.readFile`), a new function that
// returns promise is created (e.g. `fs.readFileAsync`).
Promise.promisifyAll(fs);
var to_replace =
"if ((isBrowser || isWebWorker) && !checkLoadedTimeoutId) {\n\
checkLoadedTimeoutId = setTimeout(function () {\n\
checkLoadedTimeoutId = 0;\n\
checkLoaded();\n\
}, 50);";
var replace_with =
"if (isBrowser || isWebWorker) {\n\
checkLoaded();";
gulp.task("default", function () {
// Use `fs.ensureDirAsync` to make sure the build directory
// exists.
return fs.ensureDirAsync("build").then(function () {
return fs.readFileAsync("node_modules/requirejs/require.js")
.then(function (data) {
data = data.toString();
// We use the split/join idiom to a) check that we get
// the string to be replaced exactly once and b)
// replace it. First split...
var chunks = data.split(to_replace);
// Here we check that the results of splitting the
// chunk is what we expect.
if (chunks.length < 2) {
throw new Error("did not find the pattern");
}
else if (chunks.length > 2) {
throw new Error("found the pattern more than once");
}
// We found exactly one instance of the text to
// replace, go ahead. So join...
return fs.writeFileAsync("build/require.js",
chunks.join(replace_with));
});
});
});
You need to have run npm install gulp fs-extra bluebird requirejs before running it. At any rate, you can use Gulp, you can use Grunt, or you can use any other system you want to perform a build. The points are:
You have a reproducible and automated method to patch RequireJS. If you install a new version of RequireJS with npm, when you rebuild your software the patch is applied automatically, so long as the code of RequireJS does not change in a way that prevents applying the patch. See the next point for what happens if a change prevents applying the patch.
This method is more robust than overriding setTimeout at runtime. Suppose James Burke decides in a newer version of RequireJS to rename checkLoaded to checkDone and renames the associated variables (so that checkLoadedTimeoutId becomes checkDoneTimeoutId). The gulpfile above will raise an exception when you run it again because it won't find the text to be replaced. You'll have to update the text to be replaced and the replacement so that the patch works with the new version of RequireJS. The benefit here is that you get an early warning that things have changed and that you need to review the patch. You won't have a surprise late in the game, perhaps after you've already delivered a new version of your software to clients.
The methods that override setTimeout at run time will just silently fail to do their job. They'll be looking for a function that contains checkLoadedTimeoutId, which won't exist anymore in the new version. So they will just let RequireJS behave the way it does by default. The failure will be a subtle one. (I've run RequireJS with the proposed custom versions of setTimeout with a project that loads upwards of 50 modules when not optimized. I saw no discernible difference between RequireJS using the stock setTimeout and RequireJS using a custom setTimeout.)
This method does not slow down every use of setTimeout. setTimeout is used by other code than RequireJS. No matter how you cut it, adding code in a custom replacement to setTimeout that starts looking for strings in each function passed to it will make all uses of setTimeout slower.
You can override setTimeout and check if the function passed as callback contains a variable used in that function from require.js (checkLoadedTimeoutId). If yes, call function immediately, otherwise call original setTimeout function.
(function(setTimeoutCopy) {
setTimeout = function(fn, timeout) {
if (fn.toString().indexOf("checkLoadedTimeoutId") >= 0) {
return fn();
} else {
return setTimeoutCopy.apply(null, arguments);
}
};
}(setTimeout));
Note that there are multiple issues with this code. If you pass a function to setTimeout which contains checkLoadedTimeoutId, it will be executed immediately too. Also, if require.js code is minified and variables are renamed, it won't work.
To sum up, there is no good way do this. Maybe try to find a different way to achieve what you want. Also be aware that, as Madara Uchiha said:
Changing global functions and objects is almost never a good idea.
If you really need this... Try add before loading requirejs:
function isTimeoutIgnoredFunction(fn, time) {
// you should configure this condition for safety, to exclude ignoring different timeOut calls!
return time == 50 && fn.toString().indexOf('checkLoadedTimeoutId') > -1;
}
window.setTimeoutOriginal = window.setTimeout;
window.setTimeout = function(fn, time) {
if (isTimeoutIgnoredFunction(fn, time)) {
return fn(); // or return true if you don't need to call this
} else {
return window.setTimeoutOriginal.apply(this, arguments);
}
};
Should work in last Chrome, Firefox, IE if no requirejs minify provided... You need to rewrite function "isTimeoutIgnoredFunction" for browsers you support and for minified require.js file.
To see what string you can use:
console.log((function () {
checkLoadedTimeoutId = 0;
checkLoaded();
}).toString());
But in some browsers it can just something like "Function".
If you have not many setTimeout's it can be suitable solution...
Other way to achieve it is making a ajax request of the library and patch it before load the library, but you will need to have loaded a way to make the request (vanilla js or jquery...)
The next code is an example of loading requirejs and patch it with a regexp before load it in the DOM.
$(function(){
// here you can put the url of your own hosted requirejs or a url from a CDN
var requirejsUrl = 'https://cdnjs.cloudflare.com/ajax/libs/require.js/2.1.22/require.js';
function patch(lib){
var toReplace = /if\s*\(\(isBrowser\s*\|\|\s*isWebWorker\)\s*\&\&\s*!checkLoadedTimeoutId\)\s*\{([^{}]|\{[^{}]*\})*\}/;
var by = 'if (isBrowser || isWebWorker) { checkLoaded();}';
return lib.replace(toReplace, by);
}
$.get(requirejsUrl, function(lib){
var libpatched = patch(lib);
var script=document.createElement('script');
script.innerText=libpatched;
$('body').append(script);
console.log(window.require); // requirejs patched is loaded
});
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>