This is the question:
Define a function named print which just print out the parameters it gets.
But it will not print out anything if it's called normally.
Only in a setTimeout callback will become effective.
e.g:
setTimeout(function() {
print('123'); //===> 123
});
print('456'); //===> nothing output
I have one solution but I don't think it's a good way, I rewrite the setTimeout.
I want a better solution curiously.
var print = function() {
'use strict';
var __origSetTimeout = window.setTimeout;
window.setTimeout = function(fn, delay) {
var _fn = new Function(`(${fn.toString().replace(/print\(/g, 'print.call(this,')}).call(this);`);
return __origSetTimeout.call(window, _fn.bind({
isFromSetTimeout: true
}), delay);
};
return function print(word) {
if (!this || !!this && !this.isFromSetTimeout) return;
console.log(word);
};
}.call(null);
You can use scope to solve this, for example
function A(){
let print = function(str){
console.log(str);
}
this.setTimeout = function(){
setTimeout(function(){
print('123');
}, 1000);
}
}
let a = new A();
a.setTimeout();
You could use a monkey patch for an extension of the print function with an additional check for a this object and a property for printing.
// simple function with output
function print(s) {
console.log(s);
}
// apply monkey patch
void function () {
var p = print;
print = function () {
if (this && this.timeout) {
p.apply(this, arguments);
}
}
}();
// bind additional information
setTimeout(print.bind({ timeout: true }, '123'));
print('456');
Let's assume that I have the timeout ID returned from setTimeout or setInterval.
Can I get, in some way, the original function or code, associated with it?
Something like this:
var timer_id = setTimeout(function() {
console.log('Hello Stackoverflowers!');
}, 100000);
var fn = timer_id.get_function(); // desired method
fn(); // output: 'Hello Stackoverflowers!'
You can put a wrapper around setTimeout - I just threw this one together (after a few iterations of testing...)
(function() {
var cache = {};
var _setTimeout = window.setTimeout;
var _clearTimeout = window.clearTimeout;
window.setTimeout = function(fn, delay) {
var id = _setTimeout(function() {
delete cache[id]; // ensure the map is cleared up on completion
fn();
}, delay);
cache[id] = fn;
return id;
}
window.clearTimeout = function(id) {
delete cache[id];
_clearTimeout(id);
}
window.getTimeout = function(id) {
return cache[id];
}
})();
NB: this won't work if you use a string for the callback. But no one does that, do they..?
Nor does it support passing the ES5 additional parameters to the callback function, although this would be easy to support.
var timeouts = {}; // hold the data
function makeTimeout (func, interval) {
var run = function(){
timeouts[id] = undefined;
func();
}
var id = window.setTimeout(run, interval);
timeouts[id] = func;
return id;
}
function removeTimeout (id) {
window.clearTimeout(id);
timeouts[id]=undefined;
}
function doTimeoutEarly (id) {
func = timeouts[id];
removeTimeout(id);
func();
}
var theId = makeTimeout( function(){ alert("here"); }, 10000);
console.log((timeouts[theId] || "").toString());
timeouts[theId](); // run function immediately, will still run with timer
You can store each timeout function in an object so that you can retrieve it later.
var timeout_funcs = {};
function addTimeout(func,time) {
var id = window.setTimeout(func,time);
timeout_funcs[id] = func;
return id;
}
function getTimeout(id) {
if(timeout_funcs[id])
return timeout_funcs[id];
else
return null;
}
function delTimeout(id) {
if(timeout_funcs[id]) {
window.clearTimeout(timeout_funcs[id]);
delete timeout_funcs[id];
}
}
the IDs returned from setTimeout/setInterval are just numbers, they have no properties or methods other than those that every other number would have. If you want to get that function, you can declare it first instead of using an anonymous:
var myFunc = function() {
console.log('Hello Stackoverflowers!');
};
var timer_id = setTimeout(myFunc, 100000);
myFunc(); // output: 'Hello Stackoverflowers!'
clearTimeout(timer_id); // unless you want it to fire twice
Previously I had
MyClass.prototype.method1 = function(data1) {
return this.data111.push(data1);
};
MyClass.prototype.method2 = function(i) {
var data = this.method1(i);
if (data.condition1 != null) {
data.onEvent1(this);
}
return $(data.element).someMethod123("data123");
};
MyClass.prototype.method3 = function() {
var data1 = this.method1(this._data1);
return this.someMethod123(step.data1);
};
MyClass.prototype.ended = function() {
return !!this.getState("end");
};
MyClass.prototype.getState = function(key) {
var value = $.cookie(key);
this._options.afterGetState(key, value);
return value;
};
How do I make async using callback functions? I guess it should be so:
MyClass.prototype.method1 = function(data1, callback) {
if(callback){
callback(this.data111.push(data1));
}
else{
return this.data111.push(data1);
}
};
MyClass.prototype.method2 = function(i, callback) {
var data = this.method1(i);
if (data.condition1 != null) {
data.onEvent1(this);
}
if(callback){
callback($(data.element).someMethod123("data123"));
}
else{
return $(data.element).someMethod123("data123");
}
};
MyClass.prototype.method3 = function(callback) {
var data1 = this.method1(this._data1);
if(callback){
callback(this.someMethod123(step.data1));
}
else{
return this.someMethod123(step.data1);
}
};
MyClass.prototype.ended = function(callback) {
if(callback){
callback(!!this.getState("end", /*what should be here and what should it does?*/));
}
};
MyClass.prototype.getState = function(key, callback) {
var oldThis = this;
setTimeout(function(){
value = $.cookie(key);
callback(value, oldThis);
oldThis._options.afterGetState(key, value);
},
0);
};
I definitely have missed something because I never used async functions in javascript before. So it that?
And, as I understood, to make a functions async, I basically should add one more parameter as a callback function and get rid of return, should not I?
Just pass on the callback:
MyClass.prototype.ended = function(callback) {
this.getState("end", callback);
};
You should do this in your other functions too and I'd suggest to stick to one interface. I.e. either return the value directly (if possible) or use callbacks.
Only those methods that do asynchronous tasks need callback style. There is no reason to use it for method1, method2 and method3.
getState now is the actual asynchronous method. Using ajax/setTimeout/whatever is quite obvious in here. Yet, I can spot one mistake: The callback call should always be the last statement, just as you won't do anything after a return statement. Even though you can, better call back after setting the internal options object:
…
oldThis._options.afterGetState(key, value);
callback(value, oldThis);
Now, the ended method. Since it uses the async getState, it will become asynchronous itself and you need to use callback style (notice that getState() will not return the value). So you will call getState, and when that calls back you will transform the result and pass it to your own callback:
MyClass.prototype.ended = function(callback) {
this.getState("end", function ownCallback(state) {
var result = !!state; // or whatever you need to do
callback(result);
});
};
// and if you don't need to do anything with the result, you can leave out ownCallback:
MyClass.prototype.ended = function(callback) {
this.getState("end", callback);
};
I need to create a function which can be executed only once, in each time after the first it won't be executed. I know from C++ and Java about static variables that can do the work but I would like to know if there is a more elegant way to do this?
If by "won't be executed" you mean "will do nothing when called more than once", you can create a closure:
var something = (function() {
var executed = false;
return function() {
if (!executed) {
executed = true;
// do something
}
};
})();
something(); // "do something" happens
something(); // nothing happens
In answer to a comment by #Vladloffe (now deleted): With a global variable, other code could reset the value of the "executed" flag (whatever name you pick for it). With a closure, other code has no way to do that, either accidentally or deliberately.
As other answers here point out, several libraries (such as Underscore and Ramda) have a little utility function (typically named once()[*]) that accepts a function as an argument and returns another function that calls the supplied function exactly once, regardless of how many times the returned function is called. The returned function also caches the value first returned by the supplied function and returns that on subsequent calls.
However, if you aren't using such a third-party library, but still want a utility function (rather than the nonce solution I offered above), it's easy enough to implement. The nicest version I've seen is this one posted by David Walsh:
function once(fn, context) {
var result;
return function() {
if (fn) {
result = fn.apply(context || this, arguments);
fn = null;
}
return result;
};
}
I would be inclined to change fn = null; to fn = context = null;. There's no reason for the closure to maintain a reference to context once fn has been called.
Usage:
function something() { /* do something */ }
var one_something = once(something);
one_something(); // "do something" happens
one_something(); // nothing happens
[*] Be aware, though, that other libraries, such as this Drupal extension to jQuery, may have a function named once() that does something quite different.
Replace it with a reusable NOOP (no operation) function.
// this function does nothing
function noop() {};
function foo() {
foo = noop; // swap the functions
// do your thing
}
function bar() {
bar = noop; // swap the functions
// do your thing
}
Point to an empty function once it has been called:
function myFunc(){
myFunc = function(){}; // kill it as soon as it was called
console.log('call once and never again!'); // your stuff here
};
<button onClick=myFunc()>Call myFunc()</button>
Or, like so:
var myFunc = function func(){
if( myFunc.fired ) return;
myFunc.fired = true;
console.log('called once and never again!'); // your stuff here
};
// even if referenced & "renamed"
((refToMyfunc)=>{
setInterval(refToMyfunc, 1000);
})(myFunc)
UnderscoreJs has a function that does that, underscorejs.org/#once
// Returns a function that will be executed at most one time, no matter how
// often you call it. Useful for lazy initialization.
_.once = function(func) {
var ran = false, memo;
return function() {
if (ran) return memo;
ran = true;
memo = func.apply(this, arguments);
func = null;
return memo;
};
};
Talking about static variables, this is a little bit like closure variant:
var once = function() {
if(once.done) return;
console.log('Doing this once!');
once.done = true;
};
once(); // Logs "Doing this once!"
once(); // Logs nothing
You could then reset a function if you wish:
once.done = false;
once(); // Logs "Doing this once!" again
You could simply have the function "remove itself"
function Once(){
console.log("run");
Once = undefined;
}
Once(); // run
Once(); // Uncaught TypeError: undefined is not a function
But this may not be the best answer if you don't want to be swallowing errors.
You could also do this:
function Once(){
console.log("run");
Once = function(){};
}
Once(); // run
Once(); // nothing happens
I need it to work like smart pointer, if there no elements from type A it can be executed, if there is one or more A elements the function can't be executed.
function Conditional(){
if (!<no elements from type A>) return;
// do stuff
}
var quit = false;
function something() {
if(quit) {
return;
}
quit = true;
... other code....
}
simple decorator that easy to write when you need
function one(func) {
return function () {
func && func.apply(this, arguments);
func = null;
}
}
using:
var initializer= one( _ =>{
console.log('initializing')
})
initializer() // 'initializing'
initializer() // nop
initializer() // nop
try this
var fun = (function() {
var called = false;
return function() {
if (!called) {
console.log("I called");
called = true;
}
}
})()
From some dude named Crockford... :)
function once(func) {
return function () {
var f = func;
func = null;
return f.apply(
this,
arguments
);
};
}
Reusable invalidate function which works with setInterval:
var myFunc = function (){
if (invalidate(arguments)) return;
console.log('called once and never again!'); // your stuff here
};
const invalidate = function(a) {
var fired = a.callee.fired;
a.callee.fired = true;
return fired;
}
setInterval(myFunc, 1000);
Try it on JSBin: https://jsbin.com/vicipar/edit?js,console
Variation of answer from Bunyk
Here is an example JSFiddle - http://jsfiddle.net/6yL6t/
And the code:
function hashCode(str) {
var hash = 0, i, chr, len;
if (str.length == 0) return hash;
for (i = 0, len = str.length; i < len; i++) {
chr = str.charCodeAt(i);
hash = ((hash << 5) - hash) + chr;
hash |= 0; // Convert to 32bit integer
}
return hash;
}
var onceHashes = {};
function once(func) {
var unique = hashCode(func.toString().match(/function[^{]+\{([\s\S]*)\}$/)[1]);
if (!onceHashes[unique]) {
onceHashes[unique] = true;
func();
}
}
You could do:
for (var i=0; i<10; i++) {
once(function() {
alert(i);
});
}
And it will run only once :)
Initial setup:
var once = function( once_fn ) {
var ret, is_called;
// return new function which is our control function
// to make sure once_fn is only called once:
return function(arg1, arg2, arg3) {
if ( is_called ) return ret;
is_called = true;
// return the result from once_fn and store to so we can return it multiply times:
// you might wanna look at Function.prototype.apply:
ret = once_fn(arg1, arg2, arg3);
return ret;
};
}
If your using Node.js or writing JavaScript with browserify, consider the "once" npm module:
var once = require('once')
function load (file, cb) {
cb = once(cb)
loader.load('file')
loader.once('load', cb)
loader.once('error', cb)
}
If you want to be able to reuse the function in the future then this works well based on ed Hopp's code above (I realize that the original question didn't call for this extra feature!):
var something = (function() {
var executed = false;
return function(value) {
// if an argument is not present then
if(arguments.length == 0) {
if (!executed) {
executed = true;
//Do stuff here only once unless reset
console.log("Hello World!");
}
else return;
} else {
// otherwise allow the function to fire again
executed = value;
return;
}
}
})();
something();//Hello World!
something();
something();
console.log("Reset"); //Reset
something(false);
something();//Hello World!
something();
something();
The output look like:
Hello World!
Reset
Hello World!
A simple example for turning on light only once.
function turnOnLightOnce() {
let lightOn = false;
return function () {
if (!lightOn) {
console.log("Light is not on...Turning it on for first and last time");
lightOn = true;
}
};
}
const lightOn = turnOnLightOnce();
lightOn() // Light is not on...Turning it on for first and last time
lightOn()
lightOn()
lightOn()
lightOn()
https://codesandbox.io/s/javascript-forked-ojo0i?file=/index.js
This happens due to closure in JavaScript.
function once (fn1) {
var ran = false
var memo = null
var fn = function(...args) {
if(ran) {return memo}
ran = true
memo = fn1.apply(null, args)
return memo
}
return fn
}
I'm using typescript with node and it was #I Hate Lazy's answer that inspired me. I just assigned my function to a noop function.
let printName = (name: string) => {
console.log(name)
printName = () => {}
}
printName('Sophia') // Sophia
printName('Nico') // Nothing Happens
https://jsbin.com/yuzicek/edit?js,console
FOR EVENT HANDLER
If the function is a callback for an event listener, there is already a built-in option in the addEventListner method for just executing the callback once.
It can accept 3 parameters
Type
callback
options
options is an object that has a property called once
ex:
const button = document.getElementById('button');
const callbackFunc = () => {
alert('run')
}
button.addEventListener('click', callbackFunc, { once: true })
<button id="button">Click Once</button>
Trying to use underscore "once" function:
var initialize = _.once(createApplication);
initialize();
initialize();
// Application is only created once.
http://underscorejs.org/#once
var init = function() {
console.log("logges only once");
init = false;
};
if(init) { init(); }
/* next time executing init() will cause error because now init is
-equal to false, thus typing init will return false; */
if (!window.doesThisOnce){
function myFunction() {
// do something
window.doesThisOnce = true;
};
};
If you're using Ramda, you can use the function "once".
A quote from the documentation:
once Function
(a… → b) → (a… → b)
PARAMETERS
Added in v0.1.0
Accepts a function fn and returns a function that guards invocation of fn such that fn can only ever be called once, no matter how many times the returned function is invoked. The first value calculated is returned in subsequent invocations.
var addOneOnce = R.once(x => x + 1);
addOneOnce(10); //=> 11
addOneOnce(addOneOnce(50)); //=> 11
keep it as simple as possible
function sree(){
console.log('hey');
window.sree = _=>{};
}
You can see the result
JQuery allows to call the function only once using the method one():
let func = function() {
console.log('Calling just once!');
}
let elem = $('#example');
elem.one('click', func);
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div>
<p>Function that can be called only once</p>
<button id="example" >JQuery one()</button>
</div>
Implementation using JQuery method on():
let func = function(e) {
console.log('Calling just once!');
$(e.target).off(e.type, func)
}
let elem = $('#example');
elem.on('click', func);
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div>
<p>Function that can be called only once</p>
<button id="example" >JQuery on()</button>
</div>
Implementation using native JS:
let func = function(e) {
console.log('Calling just once!');
e.target.removeEventListener(e.type, func);
}
let elem = document.getElementById('example');
elem.addEventListener('click', func);
<div>
<p>Functions that can be called only once</p>
<button id="example" >ECMAScript addEventListener</button>
</div>
Tossing my hat in the ring for fun, added advantage of memoizing
const callOnce = (fn, i=0, memo) => () => i++ ? memo : (memo = fn());
// usage
const myExpensiveFunction = () => { return console.log('joe'),5; }
const memoed = callOnce(myExpensiveFunction);
memoed(); //logs "joe", returns 5
memoed(); // returns 5
memoed(); // returns 5
...
You can use IIFE. IIFE means Immediately Invoked Function Expression and the result is to call a function only once by the time is created.
Your code will be like this:
(function () {
//The code you want to execute only one time etc...
console.log("Hello world");
})()
Additionally, this way the data in the function remains encapsulated.
Of course and you can return values from the function and stored them into a new variable, by doing:
const/let value = (function () {
//The code you want to execute only one time etc...
const x = 10;
return x;
})()
function x()
{
let a=0;
return function check()
{
if(!a++)
{
console.log("This Function will execute Once.")
return;
}
console.log("You Can't Execute it For the Second Time.")
return;
}
}
z=x()
z() //Op - This Function will execute once
z() //OP - You can't Execute it for the second time.
I find it useful to just have a simple function that just returns true once, so you can keep the side effects higher up.
let once = () => !! (once = () => false);
once() // true
once() // false
Use like this:
if (once()) {
sideEffect()
}
This exploits the fact that you can coerce an assignment expression to return true while changing the same function into a function that returns false.
If you must have it execute a function, it can be adapted using a ternary:
let once = (x) => !! (once = () => false) ? x() : false;
Now it accepts a single function as an argument. Fun fact, the second false is never reached.
// This is how function in JavaScript can be called only once
let started = false;
if (!started) {
start() { // "do something" }
}
started = true;
}
I got some methods (methA, methB ...) that need to call the same method methMain in Javascript. This method methMain then need to fetch some data and when it is done do a callback to the method that called it (methA or MethB ...).
I can successfully create a pointer/reference to a method by using what is written here: How can I pass a reference to a function, with parameters?
That solution, and all others I have seen, does not seem to work in the current scope.
This code will not work:
function TestStructure() {
this.gotData = false;
//define functions
this.methA = function (parA) { };
this.methB = function (parb) { };
this.createFunctionPointer = function (func) { };
this.createFunctionPointer = function (func /*, 0..n args */) {
var args = Array.prototype.slice.call(arguments, 1);
return function () {
var allArguments = args.concat(Array.prototype.slice.call(arguments));
return func.apply(this, allArguments);
};
};
this.methA = function (parA) {
alert('gotData: ' + this.gotData + ', parA: ' + parA);
if (this.gotData == false) {
var fp = this.createFunctionPointer(this.methA, parA);
this.methMain(fp);
return;
}
//...do stuff with data
}
this.methB = function (parB) {
alert('gotData: ' + this.gotData + ', parB: ' + parB);
if (this.gotData == false) {
var fp = this.createFunctionPointer(this.methB, parB);
this.methMain(fp);
return;
}
//...do stuff with data
}
this.methMain = function (func) {
//...get some data with ajax
this.gotData = true;
//callback to function passed in as parameter
func();
}
}
var t = new TestStructure();
t.methA('test');
When methMain do a callback to func (methA or methB) the variable this.gotData will not be set.
Is there a solution for this problem or do I need to re-think the design?
I want to do this to get data with ajax without blocking with async: false.
I am not 100% sure but I think you can solve your problem by doing
this.createFunctionPointer = function (func /*, 0..n args */) {
var args = Array.prototype.slice.call(arguments, 1);
var that = this; //<<< here
return function () {
var allArguments = args.concat(Array.prototype.slice.call(arguments));
return func.apply(that, allArguments);
//here ^^^
};
};
This will cause your partially evaluated function to be called with the same this that created the function pointer. If you want a different scope just change whatever you pass to .apply.