JavaScript question. I'm completely drawing a blank this morning.
I have four functions:
function create_account1(){ // some stuff }
function create_account2(){ // some stuff }
function create_account3(){ // some stuff }
function create_account4(){ // some stuff }
I now have need to loop through some numbers and if a criteria is met, call the appropriate function. Since I don't know which number I'm on in the loop, how can I do this?
simple example:
for( var i=1; i<=4; i++ ){
create+i+();
}
That does not work and I've tried variations, but can get it working.
Thanks for any suggestions.
johnC
Do these function live in the global context?
If so, you could use this:
for( var i=1; i<=4; i++ ){
window["create_account" + i]();
}
FYI: If you want to test this in a jsFiddle, be sure to not use the onLoad or onDomready wrap settings of jsFiddle. If you try to use them, your functions will become nested ones which obviously don't belong to the window object anymore.
Try this
function mainfunc (){
window[Array.prototype.shift.call(arguments)].apply(null, arguments);
}
The first argument you pass should be a function name to call, all the rest of the arguments would be passed onto the called function. In your case,
mainfunc('create_account1');
or
mainfun('create_account2', 'Name');
if you need to pass a name for create_account2.
Please do by this:
for( var i=1; i<=4; i++ ){
var fnName = 'create_account' + i ;
//create+i+();
window[fnName]();
}
function create_account1(){
alert('Hello');
}
You cannot call function like this. I think you're facing design problem, the proper way to do it is something like this:
function create_account(accountID) {
switch(accountID)
{
case 1:
{
//account 1 logic.
break;
}
case 2:
{
//account 2 logic.
break;
}
etc...
}
}
And you call it like this
for( var i=1; i<=4; i++ ){
create_account(i);
}
Related
I'm looking for something simple and straight forward, most of what I've pulled up on stack isn't quite what I need. I have an array that I want to loop through while calling a function after each iteration. What would that look like?
I'm assuming you're having problems with this because of the way closures are handled in Javascript. Douglas Crockford talks about this, in his book, by using the example of a function that assigns a click event handler to an array of nodes. The "intuitive" way is:
var addHandlers=function(nodes){
var i;
for(i=0; i<nodes.length;++i){
nodes[i].onClick= function {
alert (i);
};
}
};
However, this is not correct: each onClick callback will show the same value of i = nodes.length-1. This is because the value of i is not copied, but referenced in each inner function. The better way would be to create a helper function that returns a callback, something along the lines of the following:
var addHandlers = function (nodes) {
var helper = function (i){
return function (e){
alert (i);
}
}
for (int i =0; i<nodes.length();i++){
nodes [i].onClick=helper (i);
}
}
Plus, this allows you to avoid creating a function at each iteration.
var arr = [1,2,3];
for(var i = 0; i < arr.length; i++){
someFunction();
}
If you want to process one elment of the array to be used in an asynchronous funcion and then process the next next element you can do something like this;
function asynchCallback(arrayToProcess,someVar){
console.log("callback called with parameter:",someVar);
processArray(arrayToProcess);
}
function processArray(arr){
if(arr.length===0){
console.log("done");
return;
}
var someVar=tmp.splice(0,1);
setTimeout(function(){
asynchCallback(tmp,someVar[0]);
},100);
}
//send a copy of the array:
processArray([1,2,3].concat());
I am using a getJSON method to post the data I have in a database, through a for loop and into an HTML page. But I would like to the function to call different tables in my database depending on the integer the for loop is currently on, something like this:
for (var r = 0; r < 8; r++){
$.getJSON("PHP-PAGE.php?jsoncallback=?", function(table+r) {
//function stuff here
});
}
But when I try to do this, the "table+r" is flagging a syntax error. What am I doing wrong?
You are defining a function, not calling it. Between ( and ) you have to put identifiers (variable names) not expressions.
To pass data here, you need to use variables from a wider scope than the function. Since the variable is going to change (and the function is called asynchronously) you have to use a closure to do this.
function mkCallback(table) {
var foo = "table" + table;
return function () {
// function stuff that uses foo here
// foo from the time mkCallback was called to make this function
// will still be in scope
};
}
for (var r = 0; r < 8; r++){
$.getJSON("PHP-PAGE.php?jsoncallback=?", mkCallback(table+r));
}
function(table+r) { tries to create a function with table+r as a parameter, but + is not valid in a variable name. I think you instead want something like this:
for (var r = 0; r < 8; r++){
$.getJSON("PHP-PAGE.php?jsoncallback=?",
(function(currentR){
return function() {
var someVariable=table+currentR; // No idea where table came from...
//function stuff here
}
})(r));
}
As #Quentin mentioned by the time the callback is called, r will have reached its final value, hence the interesting closure.
I think what you probably want is
for (var r = 0; r < 8; r++){ //outer loop
function(tablenum){ //closure function
tablename = table+tablenum // saved reference to "table+r"
$.getJSON("PHP-PAGE.php?jsoncallback=?", function() {
//function stuff here, using tablename as the param
});
}(r)
}
This creates a closure to maintain the value of the iterated value. You can reference tablename in the callback function, and that will refer to a value equivalent to table+r
The issues with your original example
You were putting table+r as a parameter to a function you were defining, rather than an argument to one you were calling
You were trying to get the callback to reference r. But the callback won't run until after the loop has executed, so r will be 8 for all callback functions.
If you were trying to reference "table1", "table2" then you want to have "table"+r. Otherwise I assume you're referencing a table variable outside the scope of the code you showed us.
You can directly reference the variable r in your callback. Not sure what table is - the return data from the JSON call? Try the following:
for (var r = 0; r < 8; r++){
$.getJSON("PHP-PAGE.php?jsoncallback=?", function(jsonReturnData) {
//function stuff here
alert(r);
});
}
This code is supposed to pop up an alert with the number of the image when you click it:
for(var i=0; i<10; i++) {
$("#img" + i).click(
function () { alert(i); }
);
}
You can see it not working at http://jsfiddle.net/upFaJ/. I know that this is because all of the click-handler closures are referring to the same object i, so every single handler pops up "10" when it's triggered.
However, when I do this, it works fine:
for(var i=0; i<10; i++) {
(function (i2) {
$("#img" + i2).click(
function () { alert(i2); }
);
})(i);
}
You can see it working at http://jsfiddle.net/v4sSD/.
Why does it work? There's still only one i object in memory, right? Objects are always passed by reference, not copied, so the self-executing function call should make no difference. The output of the two code snippets should be identical. So why is the i object being copied 10 times? Why does it work?
I think it's interesting that this version doesn't work:
for(var i=0; i<10; i++) {
(function () {
$("#img" + i).click(
function () { alert(i); }
);
})();
}
It seems that the passing of the object as a function parameter makes all the difference.
EDIT: OK, so the previous example can be explained by primitives (i) being passed by value to the function call. But what about this example, which uses real objects?
for(var i=0; i<5; i++) {
var toggler = $("<img/>", { "src": "http://www.famfamfam.com/lab/icons/silk/icons/cross.png" });
toggler.click(function () { toggler.attr("src", "http://www.famfamfam.com/lab/icons/silk/icons/tick.png"); });
$("#container").append(toggler);
}
Not working: http://jsfiddle.net/Zpwku/
for(var i=0; i<5; i++) {
var toggler = $("<img/>", { "src": "http://www.famfamfam.com/lab/icons/silk/icons/cross.png" });
(function (t) {
t.click(function () { t.attr("src", "http://www.famfamfam.com/lab/icons/silk/icons/tick.png"); });
$("#container").append(t);
})(toggler);
}
Working: http://jsfiddle.net/YLSn6/
Most of the answers are correct in that passing an object as a function parameter breaks a closure and thus allow us to assign things to functions from within a loop. But I'd like to point out why this is the case, and it's not just a special case for closures.
You see, the way javascript passes parameters to functions is a bit different form other languages. Firstly, it seems to have two ways of doing it depending on weather it's a primitive value or an object. For primitive values it seems to pass by value and for objects it seems to pass by reference.
How javascript passes function arguments
Actually, the real explanation of what javascript does explains both situations, as well as why it breaks closures, using just a single mechanism.
What javascript does is actually it passes parameters by copy of reference. That is to say, it creates another reference to the parameter and passes that new reference into the function.
Pass by value?
Assume that all variables in javascript are references. In other languages, when we say a variable is a reference, we expect it to behave like this:
var i = 1;
function increment (n) { n = n+1 };
increment(i); // we would expect i to be 2 if i is a reference
But in javascript, it's not the case:
console.log(i); // i is still 1
That's a classic pass by value isn't it?
Pass by reference?
But wait, for objects it's a different story:
var o = {a:1,b:2}
function foo (x) {
x.c = 3;
}
foo(o);
If parameters were passed by value we'd expect the o object to be unchanged but:
console.log(o); // outputs {a:1,b:2,c:3}
That's classic pass by reference there. So we have two behaviors depending on weather we're passing a primitive type or an object.
Wait, what?
But wait a second, check this out:
var o = {a:1,b:2,c:3}
function bar (x) {
x = {a:2,b:4,c:6}
}
bar(o);
Now see what happens:
console.log(o); // outputs {a:1,b:2,c:3}
What! That's not passing by reference! The values are unchanged!
Which is why I call it pass by copy of reference. If we think about it this way, everything makes sense. We don't need to think of primitives as having special behavior when passed into a function because objects behave the same way. If we try to modify the object the variable points to then it works like pass by reference but if we try to modify the reference itself then it works like pass by value.
This also explains why closures are broken by passing a variable as a function parameter. Because the function call will create another reference that is not bound by the closure like the original variable.
Epilogue: I lied
One more thing before we end this. I said before that this unifies the behavior of primitive types and objects. Actually no, primitive types are still different:
var i = 1;
function bat (n) { n.hello = 'world' };
bat(i);
console.log(i.hello); // undefined, i is unchanged
I give up. There's no making sense of this. It's just the way it is.
It's because you are calling a function, passing it a value.
for (var i = 0; i < 10; i++) {
alert(i);
}
You expect this to alert different values, right? Because you are passing the current value of i to alert.
function attachClick(val) {
$("#img" + val).click(
function () { alert(val); }
);
}
With this function, you'd expect it to alert whatever val was passed into it, right? That also works when calling it in a loop:
for (var i = 0; i < 10; i++) {
attachClick(i);
}
This:
for (var i = 0; i < 10; i++) {
(function (val) {
$("#img" + val).click(
function () { alert(val); }
);
})(i);
}
is just an inline declaration of the above. You are declaring an anonymous function with the same characteristics as attachClick above and you call it immediately. The act of passing a value through a function parameter breaks any references to the i variable.
upvoted deceze's answer, but thought I'd try a simpler explanation. The reason the closure works is that variables in javascript are function scoped. The closure creates a new scope, and by passing the value of i in as a parameter, you are defining a local variable i in the new scope. without the closure, all of the click handlers you define are in the same scope, using the same i. the reason that your last code snippet doesn't work is because there is no local i, so all click handlers are looking to the nearest parent context with i defined.
I think the other thing that might be confusing you is this comment
Objects are always passed by reference, not copied, so the self-executing function call should make no difference.
this is true for objects, but not primitive values (numbers, for example). This is why a new local i can be defined. To demonstrate, if you did something weird like wrapping the value of i in an array, the closure would not work, because arrays are passed by reference.
// doesn't work
for(var i=[0]; i[0]<10; i[0]++) {
(function (i2) {
$("#img" + i2[0]).click(
function () { alert(i2[0]); }
);
})(i);
}
In the first example, there is only one value of i and it's the one used in the for loop. This, all event handlers will show the value of i when the for loop ends, not the desired value.
In the second example, the value of i at the time the event handler is installed is copied to the i2 function argument and there is a separate copy of that for each invocation of the function and thus for each event handler.
So, this:
(function (i2) {
$("#img" + i2).click(
function () { alert(i2); }
);
})(i);
Creates a new variable i2 that has it's own value for each separate invocation of the function. Because of closures in javascript, each separate copy of i2 is preserved for each separate event handler - thus solving your problem.
In the third example, no new copy of i is made (they all refer to the same i from the for loop) so it works the same as the first example.
Code 1 and Code 3 didn't work because i is a variable and values are changed in each loop. At the end of loop 10 will be assigned to i.
For more clear, take a look at this example,
for(var i=0; i<10; i++) {
}
alert(i)
http://jsfiddle.net/muthkum/t4Ur5/
You can see I put a alert after the loop and it will show show alert box with value 10.
This is what happening to Code 1 and Code 3.
Run the next example:
for(var i=0; i<10; i++) {
$("#img" + i).click(
function () { alert(i); }
);
}
i++;
You'll see that now, 11 is being alerted.
Therefore, you need to avoid the reference to i, by sending it as a function parameter, by it's value. You have already found the solution.
One thing that the other answers didn't mention is why this example that I gave in the question doesn't work:
for(var i=0; i<5; i++) {
var toggler = $("<img/>", { "src": "http://www.famfamfam.com/lab/icons/silk/icons/cross.png" });
toggler.click(function () { toggler.attr("src", "http://www.famfamfam.com/lab/icons/silk/icons/tick.png"); });
$("#container").append(toggler);
}
Coming back to the question months later with a better understanding of JavaScript, the reason it doesn't work can be understood as follows:
The var toggler declaration is hoisted to the top of the function call. All references to toggler are to the same actual identifier.
The closure referenced in the anonymous function is the same (not a shallow copy) of the one containing toggler, which is being updated for each iteration of the loop.
#2 is quite surprising. This alerts "5" for example:
var o;
setTimeout(function () { o = {value: 5}; }, 100);
setTimeout(function () { alert(o.value) }, 1000);
I know this kind of question gets asked alot, but I still haven't been able to find a way to make this work correctly.
The code:
function doStuff () {
for (var i = 0; i< elementsList.length; i++) {
elementsList[i].previousSibling.lastChild.addEventListener("click", function(){
toggle(elementsList[i])}, false);
}
} // ends function
function toggle (element) {
alert (element);
}
The problem is in passing variables to the toggle function. It works with the this keyword (but that sends a reference to the clicked item, which in this case is useless), but not with elementsList[i] which alerts as undefined in Firefox.
As I understood it, using anonymous functions to call a function is enough to deal with closure problems, so what have I missed?
Try:
function startOfFunction() {
for (var i = 0; i< elementsList.length; i++) {
elementsList[i].previousSibling.lastChild.addEventListener(
"click",
(function(el){return function(){toggle(el);};})(elementsList[i]),
false
);
}
} // ends function
function toggle (element) {
alert (element);
}
The Problem is, that you want to use the var i! i is available in the onClick Event, (since closure and stuff). Since you have a loop, i is counted up. Now, if you click on any of the elements, i will always be elementsList.length (since all event functions access the same i )!
using the solution of Matt will work.
As an explanation: the anonymous function you use in the for loop references the variable "i" to get the element to toggle. As anonymous functions use the "live" value of the variable, when somebody clicks the element, "i" will always be elementsList.length+1.
The code example from Matt solves this by sticking the i into another function in which it is "fixated". This always holds true:
If you iterate over elements attaching events, do not use simple anonymous functions as they screw up, but rather create a new function for each element. The more readable version of Matts answer would be:
function iterate () {
for (var i = 0; i < list.length; i++) {
// In here, i changes, so list[i] changes all the time, too. Pass it on!
list[i].addEventListener(createEventFunction(list[i]);
}
}
function createEventFunction (item) {
// In here, item is fixed as it is passed as a function parameter.
return function (event) {
alert(item);
};
}
Try:
function doStuff () {
for (var i = 0; i< elementsList.length; i++) {
(function(x) {
elementsList[x].previousSibling.lastChild.addEventListener("click", function(){
toggle(elementsList[x])}, false);
})(i);
}
} // ends function
I think it might be an issue with passing elementsList[i] around, so the above code has a closure which should help.
I'm trying to get the "click()" function to display the value of 'i' at the time I passed in the function. But its referring back to the value of 'i' after it finished. I'm drawing a blank on how to get the function to refer to the value of 'i' when I first passed the function in.
for( var i=0; i<10; i++){
var ts = $('#<span></span>').clone().click(function(){
alert(i);
});
}
NOTE:
The '#' shouldn't be there, neither should the '.clone()'
Something like this will work:
for(var i=0; i<10; i++){
(function(j) {
var ts = $('<span></span>').click(function(){
alert(j);
});
})(i);
}
You can give it a try here. Though, your creation is a bit off, I'm not sure why you'd want to create a new element just to clone it, and there's an extra # in there....I removed both of these above, but it doesn't affect the solution of an inner function.
You need to move the body of the loop to a separate function that takes i as a parameter.
You can use a normal function, like this:
for(var i=0; i<10; i++) {
makeCopy(i);
}
function makeCopy(i) {
var ts = $('#<span></span>').clone().click(function(){
alert(i);
});
}
You can also use an inline method, like this: (beware confusing syntax)
for(var i=0; i<10; i++) {
(function(i) { //Note i parameter
var ts = $('#<span></span>').clone().click(function(){
alert(i);
});
...
})(i); //Note i parameter
}