How can I best handle a situation like the following?
I have a constructor that takes a while to complete.
var Element = function Element(name){
this.name = name;
this.nucleus = {};
this.load_nucleus(name); // This might take a second.
}
var oxygen = new Element('oxygen');
console.log(oxygen.nucleus); // Returns {}, because load_nucleus hasn't finished.
I see three options, each of which seem out of the ordinary.
One, add a callback to the constructor.
var Element = function Element(name, fn){
this.name = name;
this.nucleus = {};
this.load_nucleus(name, function(){
fn(); // Now continue.
});
}
Element.prototype.load_nucleus(name, fn){
fs.readFile(name+'.json', function(err, data) {
this.nucleus = JSON.parse(data);
fn();
});
}
var oxygen = new Element('oxygen', function(){
console.log(oxygen.nucleus);
});
Two, use EventEmitter to emit a 'loaded' event.
var Element = function Element(name){
this.name = name;
this.nucleus = {};
this.load_nucleus(name); // This might take a second.
}
Element.prototype.load_nucleus(name){
var self = this;
fs.readFile(name+'.json', function(err, data) {
self.nucleus = JSON.parse(data);
self.emit('loaded');
});
}
util.inherits(Element, events.EventEmitter);
var oxygen = new Element('oxygen');
oxygen.once('loaded', function(){
console.log(this.nucleus);
});
Or three, block the constructor.
var Element = function Element(name){
this.name = name;
this.nucleus = {};
this.load_nucleus(name); // This might take a second.
}
Element.prototype.load_nucleus(name, fn){
this.nucleus = JSON.parse(fs.readFileSync(name+'.json'));
}
var oxygen = new Element('oxygen');
console.log(oxygen.nucleus)
But I haven't seen any of this done before.
What other options do I have?
Update 2:
Here is an updated example using an asynchronous factory method. N.B. this requires Node 8 or Babel if run in a browser.
class Element {
constructor(nucleus){
this.nucleus = nucleus;
}
static async createElement(){
const nucleus = await this.loadNucleus();
return new Element(nucleus);
}
static async loadNucleus(){
// do something async here and return it
return 10;
}
}
async function main(){
const element = await Element.createElement();
// use your element
}
main();
Update:
The code below got upvoted a couple of times. However I find this approach using a static method much better:
https://stackoverflow.com/a/24686979/2124586
ES6 version using promises
class Element{
constructor(){
this.some_property = 5;
this.nucleus;
return new Promise((resolve) => {
this.load_nucleus().then((nucleus) => {
this.nucleus = nucleus;
resolve(this);
});
});
}
load_nucleus(){
return new Promise((resolve) => {
setTimeout(() => resolve(10), 1000)
});
}
}
//Usage
new Element().then(function(instance){
// do stuff with your instance
});
Given the necessity to avoid blocking in Node, the use of events or callbacks isn't so strange(1).
With a slight edit of Two, you could merge it with One:
var Element = function Element(name, fn){
this.name = name;
this.nucleus = {};
if (fn) this.on('loaded', fn);
this.load_nucleus(name); // This might take a second.
}
...
Though, like the fs.readFile in your example, the core Node APIs (at least) often follow the pattern of static functions that expose the instance when the data is ready:
var Element = function Element(name, nucleus) {
this.name = name;
this.nucleus = nucleus;
};
Element.create = function (name, fn) {
fs.readFile(name+'.json', function(err, data) {
var nucleus = err ? null : JSON.parse(data);
fn(err, new Element(name, nucleus));
});
};
Element.create('oxygen', function (err, elem) {
if (!err) {
console.log(elem.name, elem.nucleus);
}
});
(1) It shouldn't take very long to read a JSON file. If it is, perhaps a change in storage system is in order for the data.
I have developed an async constructor:
function Myclass(){
return (async () => {
... code here ...
return this;
})();
}
(async function() {
let s=await new Myclass();
console.log("s",s)
})();
async returns a promise
arrow functions pass 'this' as is
it is possible to return something else when doing new (you still get a new empty object in this variable. if you call the function without new. you get the original this. like maybe window or global or its holding object).
it is possible to return the return value of called async function using await.
to use await in normal code, need to wrap the calls with an async anonymous function, that is called instantly. (the called function returns promise and code continues)
my 1st iteration was:
maybe just add a callback
call an anonymous async function,
then call the callback.
function Myclass(cb){
var asynccode=(async () => {
await this.something1();
console.log(this.result)
})();
if(cb)
asynccode.then(cb.bind(this))
}
my 2nd iteration was:
let's try with a promise instead of a callback.
I thought to myself: strange a promise returning a promise, and it worked. .. so the next version is just a promise.
function Myclass(){
this.result=false;
var asynccode=(async () => {
await new Promise (resolve => setTimeout (()=>{this.result="ok";resolve()}, 1000))
console.log(this.result)
return this;
})();
return asynccode;
}
(async function() {
let s=await new Myclass();
console.log("s",s)
})();
callback-based for old javascript
function Myclass(cb){
var that=this;
var cb_wrap=function(data){that.data=data;cb(that)}
getdata(cb_wrap)
}
new Myclass(function(s){
});
One thing you could do is preload all the nuclei (maybe inefficient; I don't know how much data it is). The other, which I would recommend if preloading is not an option, would involve a callback with a cache to save loaded nuclei. Here is that approach:
Element.nuclei = {};
Element.prototype.load_nucleus = function(name, fn){
if ( name in Element.nuclei ) {
this.nucleus = Element.nuclei[name];
return fn();
}
fs.readFile(name+'.json', function(err, data) {
this.nucleus = Element.nuclei[name] = JSON.parse(data);
fn();
});
}
This is a bad code design.
The main problem is in the callback your instance it's not still execute the "return", this is what I mean
var MyClass = function(cb) {
doAsync(function(err) {
cb(err)
}
return {
method1: function() { },
method2: function() { }
}
}
var _my = new MyClass(function(err) {
console.log('instance', _my) // < _my is still undefined
// _my.method1() can't run any methods from _my instance
})
_my.method1() // < it run the function, but it's not yet inited
So, the good code design is to explicitly call the "init" method (or in your case "load_nucleus") after instanced the class
var MyClass = function() {
return {
init: function(cb) {
doAsync(function(err) {
cb(err)
}
},
method1: function() { },
method2: function() { }
}
}
var _my = new MyClass()
_my.init(function(err) {
if(err) {
console.error('init error', err)
return
}
console.log('inited')
// _my.method1()
})
I extract out the async portions into a fluent method. By convention I call them together.
class FooBar {
constructor() {
this.foo = "foo";
}
async create() {
this.bar = await bar();
return this;
}
}
async function bar() {
return "bar";
}
async function main() {
const foobar = await new FooBar().create(); // two-part constructor
console.log(foobar.foo, foobar.bar);
}
main(); // foo bar
I tried a static factory approach wrapping new FooBar(), e.g. FooBar.create(), but it didn't play well with inheritance. If you extend FooBar into FooBarChild, FooBarChild.create() will still return a FooBar. Whereas with my approach new FooBarChild().create() will return a FooBarChild and it's easy to setup an inheritance chain with create().
You can run constructor function with async functions synchronously via nsynjs. Here is an example to illustrate:
index.js (main app logic):
var nsynjs = require('nsynjs');
var modules = {
MyObject: require('./MyObject')
};
function synchronousApp(modules) {
try {
var myObjectInstance1 = new modules.MyObject('data1.json');
var myObjectInstance2 = new modules.MyObject('data2.json');
console.log(myObjectInstance1.getData());
console.log(myObjectInstance2.getData());
}
catch (e) {
console.log("Error",e);
}
}
nsynjs.run(synchronousApp,null,modules,function () {
console.log('done');
});
MyObject.js (class definition with slow constructor):
var nsynjs = require('nsynjs');
var synchronousCode = function (wrappers) {
var config;
// constructor of MyObject
var MyObject = function(fileName) {
this.data = JSON.parse(wrappers.readFile(nsynjsCtx, fileName).data);
};
MyObject.prototype.getData = function () {
return this.data;
};
return MyObject;
};
var wrappers = require('./wrappers');
nsynjs.run(synchronousCode,{},wrappers,function (m) {
module.exports = m;
});
wrappers.js (nsynjs-aware wrapper around slow functions with callbacks):
var fs=require('fs');
exports.readFile = function (ctx,name) {
var res={};
fs.readFile( name, "utf8", function( error , configText ){
if( error ) res.error = error;
res.data = configText;
ctx.resume(error);
} );
return res;
};
exports.readFile.nsynjsHasCallback = true;
Full set of files for this example could be found here: https://github.com/amaksr/nsynjs/tree/master/examples/node-async-constructor
Related
I was wondering what the best approach is for configuring a module export. "async.function" in the example below could be a FS or HTTP request, simplified for the sake of the example:
Here's example code (asynmodule.js):
var foo = "bar"
async.function(function(response) {
foo = "foobar";
// module.exports = foo; // having the export here breaks the app: foo is always undefined.
});
// having the export here results in working code, but without the variable being set.
module.exports = foo;
How can I export the module only once the async callback has been executed?
edit
a quick note on my actual use-case: I'm writing a module to configure nconf (https://github.com/flatiron/nconf) in an fs.exists() callback (i.e. it will parse a config file and set up nconf).
Your export can't work because it is outside the function while the foodeclaration is inside. But if you put the export inside, when you use your module you can't be sure the export was defined.
The best way to work with an ansync system is to use callback. You need to export a callback assignation method to get the callback, and call it on the async execution.
Example:
var foo, callback;
async.function(function(response) {
foo = "foobar";
if( typeof callback == 'function' ){
callback(foo);
}
});
module.exports = function(cb){
if(typeof foo != 'undefined'){
cb(foo); // If foo is already define, I don't wait.
} else {
callback = cb;
}
}
Here async.function is just a placeholder to symbolise an async call.
In main
var fooMod = require('./foo.js');
fooMod(function(foo){
//Here code using foo;
});
Multiple callback way
If your module need to be called more than once you need to manage an array of callback:
var foo, callbackList = [];
async.function(function(response) {
foo = "foobar";
// You can use all other form of array walk.
for(var i = 0; i < callbackList.length; i++){
callbackList[i](foo)
}
});
module.exports = function(cb){
if(typeof foo != 'undefined'){
cb(foo); // If foo is already define, I don't wait.
} else {
callback.push(cb);
}
}
Here async.function is just a placeholder to symbolise an async call.
In main
var fooMod = require('./foo.js');
fooMod(function(foo){
//Here code using foo;
});
Promise way
You can also use Promise to solve that. This method support multiple call by the design of the Promise:
var foo, callback;
module.exports = new Promise(function(resolve, reject){
async.function(function(response) {
foo = "foobar"
resolve(foo);
});
});
Here async.function is just a placeholder to symbolise an async call.
In main
var fooMod = require('./foo.js').then(function(foo){
//Here code using foo;
});
See Promise documentation
An ES7 approach would be an immediatly invoked async function in module.exports :
module.exports = (async function(){
//some async initiallizers
//e.g. await the db module that has the same structure like this
var db = await require("./db");
var foo = "bar";
//resolve the export promise
return {
foo
};
})()
This can be required with await later:
(async function(){
var foo = await require("./theuppercode");
console.log(foo);
})();
ES6 answer using promises:
const asyncFunc = () => {
return new Promise((resolve, reject) => {
// Where someAsyncFunction takes a callback, i.e. api call
someAsyncFunction(data => {
resolve(data)
})
})
}
export default asyncFunc
...
import asyncFunc from './asyncFunc'
asyncFunc().then(data => { console.log(data) })
Or you could return the Promise itself directly:
const p = new Promise(...)
export default p
...
import p from './asyncModule'
p.then(...)
Another approach would be wrapping the variable inside an object.
var Wrapper = function(){
this.foo = "bar";
this.init();
};
Wrapper.prototype.init = function(){
var wrapper = this;
async.function(function(response) {
wrapper.foo = "foobar";
});
}
module.exports = new Wrapper();
If the initializer has error, at least you still get the uninitialized value instead of hanging callback.
You can also make use of Promises:
some-async-module.js
module.exports = new Promise((resolve, reject) => {
setTimeout(resolve.bind(null, 'someValueToBeReturned'), 2000);
});
main.js
var asyncModule = require('./some-async-module');
asyncModule.then(promisedResult => console.log(promisedResult));
// outputs 'someValueToBeReturned' after 2 seconds
The same can happen in a different module and will also resolve as expected:
in-some-other-module.js
var asyncModule = require('./some-async-module');
asyncModule.then(promisedResult => console.log(promisedResult));
// also outputs 'someValueToBeReturned' after 2 seconds
Note that the promise object is created once then it's cached by node. Each require('./some-async-module') will return the same object instance (promise instance in this case).
Other answers seemed to be partial answers and didn't work for me. This seems to be somewhat complete:
some-module.js
var Wrapper = function(){
this.callbacks = [];
this.foo = null;
this.init();
};
Wrapper.prototype.init = function(){
var wrapper = this;
async.function(function(response) {
wrapper.foo = "foobar";
this.callbacks.forEach(function(callback){
callback(null, wrapper.foo);
});
});
}
Wrapper.prototype.get = function(cb) {
if(typeof cb !== 'function') {
return this.connection; // this could be null so probably just throw
}
if(this.foo) {
return cb(null, this.foo);
}
this.callbacks.push(cb);
}
module.exports = new Wrapper();
main.js
var wrapper = require('./some-module');
wrapper.get(function(foo){
// foo will always be defined
});
main2.js
var wrapper = require('./some-module');
wrapper.get(function(foo){
// foo will always be defined in another script
});
I was wondering what the best approach is for configuring a module export. "async.function" in the example below could be a FS or HTTP request, simplified for the sake of the example:
Here's example code (asynmodule.js):
var foo = "bar"
async.function(function(response) {
foo = "foobar";
// module.exports = foo; // having the export here breaks the app: foo is always undefined.
});
// having the export here results in working code, but without the variable being set.
module.exports = foo;
How can I export the module only once the async callback has been executed?
edit
a quick note on my actual use-case: I'm writing a module to configure nconf (https://github.com/flatiron/nconf) in an fs.exists() callback (i.e. it will parse a config file and set up nconf).
Your export can't work because it is outside the function while the foodeclaration is inside. But if you put the export inside, when you use your module you can't be sure the export was defined.
The best way to work with an ansync system is to use callback. You need to export a callback assignation method to get the callback, and call it on the async execution.
Example:
var foo, callback;
async.function(function(response) {
foo = "foobar";
if( typeof callback == 'function' ){
callback(foo);
}
});
module.exports = function(cb){
if(typeof foo != 'undefined'){
cb(foo); // If foo is already define, I don't wait.
} else {
callback = cb;
}
}
Here async.function is just a placeholder to symbolise an async call.
In main
var fooMod = require('./foo.js');
fooMod(function(foo){
//Here code using foo;
});
Multiple callback way
If your module need to be called more than once you need to manage an array of callback:
var foo, callbackList = [];
async.function(function(response) {
foo = "foobar";
// You can use all other form of array walk.
for(var i = 0; i < callbackList.length; i++){
callbackList[i](foo)
}
});
module.exports = function(cb){
if(typeof foo != 'undefined'){
cb(foo); // If foo is already define, I don't wait.
} else {
callback.push(cb);
}
}
Here async.function is just a placeholder to symbolise an async call.
In main
var fooMod = require('./foo.js');
fooMod(function(foo){
//Here code using foo;
});
Promise way
You can also use Promise to solve that. This method support multiple call by the design of the Promise:
var foo, callback;
module.exports = new Promise(function(resolve, reject){
async.function(function(response) {
foo = "foobar"
resolve(foo);
});
});
Here async.function is just a placeholder to symbolise an async call.
In main
var fooMod = require('./foo.js').then(function(foo){
//Here code using foo;
});
See Promise documentation
An ES7 approach would be an immediatly invoked async function in module.exports :
module.exports = (async function(){
//some async initiallizers
//e.g. await the db module that has the same structure like this
var db = await require("./db");
var foo = "bar";
//resolve the export promise
return {
foo
};
})()
This can be required with await later:
(async function(){
var foo = await require("./theuppercode");
console.log(foo);
})();
ES6 answer using promises:
const asyncFunc = () => {
return new Promise((resolve, reject) => {
// Where someAsyncFunction takes a callback, i.e. api call
someAsyncFunction(data => {
resolve(data)
})
})
}
export default asyncFunc
...
import asyncFunc from './asyncFunc'
asyncFunc().then(data => { console.log(data) })
Or you could return the Promise itself directly:
const p = new Promise(...)
export default p
...
import p from './asyncModule'
p.then(...)
Another approach would be wrapping the variable inside an object.
var Wrapper = function(){
this.foo = "bar";
this.init();
};
Wrapper.prototype.init = function(){
var wrapper = this;
async.function(function(response) {
wrapper.foo = "foobar";
});
}
module.exports = new Wrapper();
If the initializer has error, at least you still get the uninitialized value instead of hanging callback.
You can also make use of Promises:
some-async-module.js
module.exports = new Promise((resolve, reject) => {
setTimeout(resolve.bind(null, 'someValueToBeReturned'), 2000);
});
main.js
var asyncModule = require('./some-async-module');
asyncModule.then(promisedResult => console.log(promisedResult));
// outputs 'someValueToBeReturned' after 2 seconds
The same can happen in a different module and will also resolve as expected:
in-some-other-module.js
var asyncModule = require('./some-async-module');
asyncModule.then(promisedResult => console.log(promisedResult));
// also outputs 'someValueToBeReturned' after 2 seconds
Note that the promise object is created once then it's cached by node. Each require('./some-async-module') will return the same object instance (promise instance in this case).
Other answers seemed to be partial answers and didn't work for me. This seems to be somewhat complete:
some-module.js
var Wrapper = function(){
this.callbacks = [];
this.foo = null;
this.init();
};
Wrapper.prototype.init = function(){
var wrapper = this;
async.function(function(response) {
wrapper.foo = "foobar";
this.callbacks.forEach(function(callback){
callback(null, wrapper.foo);
});
});
}
Wrapper.prototype.get = function(cb) {
if(typeof cb !== 'function') {
return this.connection; // this could be null so probably just throw
}
if(this.foo) {
return cb(null, this.foo);
}
this.callbacks.push(cb);
}
module.exports = new Wrapper();
main.js
var wrapper = require('./some-module');
wrapper.get(function(foo){
// foo will always be defined
});
main2.js
var wrapper = require('./some-module');
wrapper.get(function(foo){
// foo will always be defined in another script
});
Phew, even the question was hard to write. Here's the problem: I have a "game", more like a random simulator, which needs to choose a random action from an array of actions, like this one:
actions = [ Action1, Action2, Action3 ]
I have actions written as classes inheriting from the Action parent class:
function Action() {
this.targets = [];
this.used = [];
this.execute = function(player) {
doStuff();
return whatever;
};
}
//btw the below I've seen in a JS OOP tutorial but it doesn't work and I have to implement init() in every child action
Action.init = function(player) {
var a = new this.constructor();
return a.execute(player);
};
Action.checkRequirements = function() {
return true;
};
Action1.prototype = new Action();
Action1.prototype.constructor = Action1;
function Action1 {
this.execute = function(player) {
doStuff();
return whatever;
}
}
Action1.init = function(player) {
var a = new Action1();
return a.execute(player);
}
So what I'm doing to execute an action and get its results is var foo = actions.getRandomVal().init(); (getRandomVal is a simple custom script that returns a random value from the array) It works well, creates the object instance which properly inherits all properties and methods, executes the exec() method and returns its results... but now I have a checkRequirements() method which I want to implement in like 10% of the 100+ actions I wish to do, and I want it to simply be inherited from the Action class so that when it is not implemented in the child class it simply returns true and I don't have an idea how. If I do var a = actions.getRandomVal(); and then a.checkRequirements(); it throws an exception that a.checkRequirements is not a function.
PS: this is a relatively small non-profit project for a (large) group of friends, I don't need it to work in every browser, it needs to work in Chrome and I can just tell them to use Chrome for it.
Since you only need to work with Chrome, I'd suggest to use ES6 class syntax which does all the inheritance properly, without the chance to mess up. This includes your Action1 constructor to inherit properties ("static class members") from the Action constructor as you'd expect.
class Action {
constructor() {
this.targets = [];
this.used = [];
}
execute(player) {
doStuff();
return whatever;
}
static init(player) {
var a = new this(); // no .constructor
return a.execute(player);
}
static checkRequirements() {
return true;
}
}
class Action1 {
execute(player) {
doOtherStuff();
return whateverelse;
}
}
It looks to me like you're calling checkRequirements() on an instance:
a.checkRequirements();
But it's implemented statically:
Action.checkRequirements = function() {
return true;
};
You probably want to bind this function to the prototype, so change the code above to this:
Action.prototype.checkRequirements = function() {
return true;
};
Then when you want to override this in a derived type, like Action1, you can do this:
Action1.prototype.checkRequirements = function () {
return (whatever);
}
As per comments, my guess is you want something like this...
// base Action type providing basic implementation
// Wrapped in an IIFE to prevent global scope pollution
// All functions are prototype bound to allow prototypical inheritance.
var Action = (function () {
function Action() {
this.targets = [];
this.used = [];
};
Action.prototype.doStuff = function () {
return;
}
Action.prototype.execute = function (player) {
this.doStuff();
return "whatever";
}
Action.prototype.checkRequirements = function () {
return "foo";
}
return Action;
})();
var Action1 = (function () {
Action1.prototype = new Action();
Action1.prototype.constructor = Action1;
function Action1() {
}
Action1.prototype.checkRequirements = function () {
// Super call
return Action.prototype.checkRequirements.call(this);
}
return Action1;
})();
var Action2 = (function () {
Action2.prototype = new Action();
Action2.prototype.constructor = Action2;
function Action2() {
}
Action2.prototype.checkRequirements = function () {
return "bar";
}
return Action2;
})();
// Set up array.
var array = [Action1, Action2];
// Create instances (this is where you would pick at random)
var a1 = new array[0]();
var a2 = new array[1]();
// var aofn = new array[rnd]();
// Tests
alert(a1.checkRequirements()); // Should "foo" because it called super (Action).
alert(a2.checkRequirements()); // Should "bar" because it's overridden.
Check it out on TypeScript Playground
I was wondering what the best approach is for configuring a module export. "async.function" in the example below could be a FS or HTTP request, simplified for the sake of the example:
Here's example code (asynmodule.js):
var foo = "bar"
async.function(function(response) {
foo = "foobar";
// module.exports = foo; // having the export here breaks the app: foo is always undefined.
});
// having the export here results in working code, but without the variable being set.
module.exports = foo;
How can I export the module only once the async callback has been executed?
edit
a quick note on my actual use-case: I'm writing a module to configure nconf (https://github.com/flatiron/nconf) in an fs.exists() callback (i.e. it will parse a config file and set up nconf).
Your export can't work because it is outside the function while the foodeclaration is inside. But if you put the export inside, when you use your module you can't be sure the export was defined.
The best way to work with an ansync system is to use callback. You need to export a callback assignation method to get the callback, and call it on the async execution.
Example:
var foo, callback;
async.function(function(response) {
foo = "foobar";
if( typeof callback == 'function' ){
callback(foo);
}
});
module.exports = function(cb){
if(typeof foo != 'undefined'){
cb(foo); // If foo is already define, I don't wait.
} else {
callback = cb;
}
}
Here async.function is just a placeholder to symbolise an async call.
In main
var fooMod = require('./foo.js');
fooMod(function(foo){
//Here code using foo;
});
Multiple callback way
If your module need to be called more than once you need to manage an array of callback:
var foo, callbackList = [];
async.function(function(response) {
foo = "foobar";
// You can use all other form of array walk.
for(var i = 0; i < callbackList.length; i++){
callbackList[i](foo)
}
});
module.exports = function(cb){
if(typeof foo != 'undefined'){
cb(foo); // If foo is already define, I don't wait.
} else {
callback.push(cb);
}
}
Here async.function is just a placeholder to symbolise an async call.
In main
var fooMod = require('./foo.js');
fooMod(function(foo){
//Here code using foo;
});
Promise way
You can also use Promise to solve that. This method support multiple call by the design of the Promise:
var foo, callback;
module.exports = new Promise(function(resolve, reject){
async.function(function(response) {
foo = "foobar"
resolve(foo);
});
});
Here async.function is just a placeholder to symbolise an async call.
In main
var fooMod = require('./foo.js').then(function(foo){
//Here code using foo;
});
See Promise documentation
An ES7 approach would be an immediatly invoked async function in module.exports :
module.exports = (async function(){
//some async initiallizers
//e.g. await the db module that has the same structure like this
var db = await require("./db");
var foo = "bar";
//resolve the export promise
return {
foo
};
})()
This can be required with await later:
(async function(){
var foo = await require("./theuppercode");
console.log(foo);
})();
ES6 answer using promises:
const asyncFunc = () => {
return new Promise((resolve, reject) => {
// Where someAsyncFunction takes a callback, i.e. api call
someAsyncFunction(data => {
resolve(data)
})
})
}
export default asyncFunc
...
import asyncFunc from './asyncFunc'
asyncFunc().then(data => { console.log(data) })
Or you could return the Promise itself directly:
const p = new Promise(...)
export default p
...
import p from './asyncModule'
p.then(...)
Another approach would be wrapping the variable inside an object.
var Wrapper = function(){
this.foo = "bar";
this.init();
};
Wrapper.prototype.init = function(){
var wrapper = this;
async.function(function(response) {
wrapper.foo = "foobar";
});
}
module.exports = new Wrapper();
If the initializer has error, at least you still get the uninitialized value instead of hanging callback.
You can also make use of Promises:
some-async-module.js
module.exports = new Promise((resolve, reject) => {
setTimeout(resolve.bind(null, 'someValueToBeReturned'), 2000);
});
main.js
var asyncModule = require('./some-async-module');
asyncModule.then(promisedResult => console.log(promisedResult));
// outputs 'someValueToBeReturned' after 2 seconds
The same can happen in a different module and will also resolve as expected:
in-some-other-module.js
var asyncModule = require('./some-async-module');
asyncModule.then(promisedResult => console.log(promisedResult));
// also outputs 'someValueToBeReturned' after 2 seconds
Note that the promise object is created once then it's cached by node. Each require('./some-async-module') will return the same object instance (promise instance in this case).
Other answers seemed to be partial answers and didn't work for me. This seems to be somewhat complete:
some-module.js
var Wrapper = function(){
this.callbacks = [];
this.foo = null;
this.init();
};
Wrapper.prototype.init = function(){
var wrapper = this;
async.function(function(response) {
wrapper.foo = "foobar";
this.callbacks.forEach(function(callback){
callback(null, wrapper.foo);
});
});
}
Wrapper.prototype.get = function(cb) {
if(typeof cb !== 'function') {
return this.connection; // this could be null so probably just throw
}
if(this.foo) {
return cb(null, this.foo);
}
this.callbacks.push(cb);
}
module.exports = new Wrapper();
main.js
var wrapper = require('./some-module');
wrapper.get(function(foo){
// foo will always be defined
});
main2.js
var wrapper = require('./some-module');
wrapper.get(function(foo){
// foo will always be defined in another script
});
Let's say I have a function named fna() that does a simple thing such as:
var fna = function(ar) {
console.log("argument: ", ar);
return "return value is argument too: " + ar;
}
fna() is coded by some other developer and I can't access to it. He didn't bother casting any events and when it is called, I have to be aware of it. Hopefully, his method is accessible by window.fna().
I want some additional code to be executed. Let's say, add this console.log
var fna = function(ar) {
console.log("Hola, I am some additional stuff being rewired");
console.log("argument:", ar);
return "return value is argument too: " + ar;
}
And I want this to be executed even when called from fnb() by some other part of the code.
var fnb = function() {
return fna("Bonjour, I am fnb and I call fna");
}
Here is a way I found, using the utils.rewire() method. utils is just some utility belt, and it could be added to your favorite framework as a plugin. Unfortunately, it only works on Firefox.
var utils = utils || {};
// Let's rewire a function. i.e. My.super.method()
utils.rewire = function(functionFullName, callback) {
var rewired = window[functionFullName];
console.log("%s() is being rewired", functionFullName)
window[functionFullName] = function() {
callback();
return rewired.apply(this, arguments);
}
}
Use it like this.
utils.rewire("fna",function(){
console.log("Hola, I am some additional stuffs being rewired");
});
This seems to work such as shown in this jsbin, but (and here is my question:) How do I rewire obja.fna()?
var obja = {
fna = function(ar) {
console.log("argument:", ar);
return "return value is argument too: " + ar;
}
};
I cannot make it work to rewire the some.object.method() method.
Extra bonus question: Is there a more cleaner way to do this? Out-of-the-box clean concise and magic library?
Refactor rewire into a rewireMethod function which acts on any given object:
var utils = utils || {};
utils.rewireMethod = function (obj, functionName, prefunc) {
var original = obj[functionName];
obj[functionName] = function () {
prefunc();
return original.apply(this, arguments);
};
};
Note that rewire can now be written as:
utils.rewire = function (functionName, prefunc) {
utils.rewireMethod(window, functionName, prefunc);
};
Then you just call it as:
utils.rewireMethod(obja, "fna", function () {
console.log("Hola, I am some additional stuff being rewired");
});
Note that nothing special is required if you have a method like window.ideeli.Search.init(). In that case, the object is window.ideeli.Search, and the method name is init:
utils.rewireMethod(window.ideeli.Search, "init", function () {
console.log("Oh yeah, nested objects.");
});
Add a parameter to rewire that is the object containing the function. If it's a global function, pass in window.
var utils = utils || {};
// let's rewire a function. i.e. My.super.method()
utils.rewire = function(object, functionName, callback) {
var rewired = object[functionName];
console.log("%s() is being rewired", functionName)
object[functionName] = function() {
callback();
return rewired.apply(this, arguments);
}
}
utils.rewire(some.object, "method", function(){} );
You can simply use a closure to create a generic hook function that allows you to specify another function to be called immediately before or after the original function:
function hookFunction(fn, preFn, postFn) {
function hook() {
var retVal;
if (preFn) {
preFn.apply(this, arguments);
}
retVal = fn.apply(this, arguments);
if (postFn) {
postFn.apply(this, arguments);
}
return retVal;
}
return hook;
}
So, for any function that you want to hook, you just call hookFunction and pass it the function you want to hook and then an optional pre and post function or yours. The pre and post function are passed the same arguments that the original function was.
So, if your original function was this:
var fna = function(ar) {
console.log("argument:",ar);
return "return value is argument too:"+ar;
}
And, you want something to happen every time that function is called right before it's called, you would do this:
fna = hookFunction(fna, function() {
console.log("Hola, I am some additional stuff being rewired right before");
});
or if you wanted it to happen right after the original was called, you could do it like this:
fna = hookFunction(fna, null, function() {
console.log("Hola, I am some additional stuff being rewired right after");
});
Working demo: http://jsfiddle.net/jfriend00/DMgn6/
This can be used with methods on objects and arbitrary nesting levels of objects and methods.
var myObj = function(msg) {
this.greeting = msg;
};
myObj.prototype = {
test: function(a) {
log("myObj.test: " + this.greeting);
}
}
var x = new myObj("hello");
x.test = hookFunction(x.test, mypreFunc2, myPostFunc2);
x.test("hello");
Based on Claudiu's answer, which seems to be the most appreciated way, here is a solution using a for loop and proxying the context... But still, I find this ugly.
var utils = utils || {};
// Let's rewire a function. i.e. My.super.method()
utils.rewire = function(method, callback) {
var obj = window;
var original = function() {};
var tree = method.split(".");
var fun = tree.pop();
console.log(tree);
// Parse through the hierarchy
for (var i = 0; i < tree.length; i++) {
obj = obj[tree[i]];
}
if(typeof(obj[fun]) === "function") {
original = obj[fun];
}
var cb = callback.bind(obj);
obj[fun] = function(ar) {
cb();
return original.apply(this, arguments);
}
}
Well, this looks strange. Consider this
function wrap(fn, wrapper) {
return function() {
var a = arguments;
return wrapper(function() { return fn.apply(this, a) })
}
}
Example:
function foo(a, b) {
console.log([a, b])
return a + b
}
bar = wrap(foo, function(original) {
console.log("hi")
var ret = original()
console.log("there")
return ret
})
console.log(bar(11,22))
Result:
hi
[11, 22]
there
33
To wrap object methods, just bind them:
obj = {
x: 111,
foo: function(a, b) {
console.log([a, b, this.x])
}
}
bar = wrap(obj.foo.bind(obj), function(fn) {
console.log("hi")
return fn()
})