I know there are lots of answers already about this query but my question is on the receiving function.
Say I have three functions:
function A(a){
var j = getList(a);
j != null? process.apply(null,j): null;
}
function getList(a){
// returns an array like array[][] with no definite size
}
// I know this function should accept multiple arguments but I want the whole array to be passed
function process(j){
// I want to loop the array here but it seems like
// the argument passed is value of array[0][0]
//
}
I know that in c, it should be:
function process(j[][]){
And python has no problem just passing j directly. Now, javascript makes me wonder how to implement this. Your help is very much appreciated.
Apply takes an array of arguments, but you are passing a single argument as an array.
There are a couple ways to solve this, one way is I just wrapped j with [j] in the apply, that way its actually passing the array as the first element in the array of arguments.
I'm sure there is a better way to explain that, but I couldn't think of it.
function A(a){
var j = getList(a);
j != null? process.apply(null,[j]): null;
}
function getList(a){
// returns an array like array[][] with no definite size
return a;
}
// I know this function should accept multiple arguments but I want the whole array to be passed
function process(j){
console.log(j);
// I want to loop the array here but it seems like
// the argument passed is value of array[0][0]
//
}
A(["1","2"]);
While attempt to read "secrets of the javascript Ninja"(ok, so perhaps I am not qualify to read this book just yet) but I see below code and I understand what code is doing but part I really don't understand is where function(index) is being called. Is index arbitrary terms or some sort of javascript to indicate generic index?
<script type="text/javascript">
function forEach(list,callback) {
for (var n = 0; n < list.length; n++) {
callback.call(list[n],n);
}
}
var weapons = ['shuriken','katana','nunchucks'];
forEach(
weapons,
function(index){
function
 assert(this == weapons [index],
"Got the expected value of " + weapons [index]);
} );
</script>
It is passed as the second argument to forEach.
forEach(list,callback)
… to a variable called callback.
So it is called here:
callback.call(list[n],n);
via the call method
Is index arbitrary terms or some sort of javascript to indicate generic index?
It's an argument name. You defined the name yourself. The value it gets passed is determined when you call the function.
index it's an alias for the variable that will be available inside the execution context of the callback, hence available to the caller, the name could be whatever.
this is an example of a function used as a first class function (a crucial concept in javascript), that is pass them around as objects to define custom behaviour.
I have an assignment to count repeated strings base on a Heap's Algorithm Permutation.The first thing I want to do is output the swapped strings, I found this code from jake's answer Can someone please help me understand recursion within this code in a loop? The output of this function are swapped strings.
function permAlone(string) {
var arr = string.split(''), // Turns the input string into a letter array.
permutations = []; // results
function swap(a, b) {
debugger; // This function will simply swap positions a and b inside the input array.
var tmp = arr[a];
arr[a] = arr[b];
arr[b] = tmp;
}
function gen(n) {
debugger;
if (n === 1) {
var x =arr.join('');
permutations.push(x);
} else {
for (var i = 0; i != n; i++) { // how does this loop executes within the call stack?
gen(n - 1);
debugger;
swap(n % 2 ? 0 : i, n - 1); // i don't understand this part. i understand the swap function, but I don't get how indexes are swapped here
}
}
}
gen(arr.length);
return permutations;
}
permAlone('xyz'); // output -> ["xyz","yxz","zxy","xzy","yzx","zyx"]
I have been experimenting it on debugger but still can't get what's happening.
I'm not sure what you mean by
understand recursion within this code in a loop
If you mean you want to see the algorithm in a loop form rather than a recursion version you can see them one by side in pseudocode in the wikipedia page here.
For your questions within the code:
how does this loop executes within the call stack?
You are right to refer to the call stack, and this is a general question regarding recursion. If you don't understand how recursion works with the stack you can refer to this really nice and simple video that demonstrates recursive calls using factorial calculation in java (start around min 4:00).
The line you look at is no different than any other line in the recursive function. We start by defining i and assigning the value 0 to it. We continue to check if it satisfies the condition of the for loop. If it does we step into the loop and execute the first line inside the loop which is the recursive call. Inside the recursive call we have a new stack frame which has no knowledge of the i variable we defined before executing the recursive call, because it is a local variable. So when we get to the loop in the new call we define a new variable i, assigning it 0 at first and incrementing it as the loop repeats in this stack frame/call instance. When this call finishes we delete the stack frame and resume to the previous stack frame (the one we started with) where i=0 still, and we continue to the next line.
All the calls have access to the arr and permutations variables since the function is defined in the same scope as the variables (inside the function permAlone) so within each call - no matter what the stack frame we are in, the changes made to those are made to the same instances. That's why every push done to permutations adds to the existing results and will be there when the function returns the variable at the end.
i don't understand this part. i understand the swap function, but I don't get how indexes are swapped here
Indexes are not swapped here. It is merely a call for the swap function with the correct indices.
swap(n % 2 ? 0 : i, n - 1);
is just
swap(a, b);
with
a = n% 2 ? 0 : i;
b = n - 1;
If the a part is what confuses you, then this is a use of the ternary operator for conditional value. That is, it's symbols used to form an expression that is evaluated differently according to the circumstances. The use is by
<<i>boolean epression</i>> ? <<i>value-if-true</i>> : <<i>value-if-false</i>>
to evaluate the above, first <boolean expression> is evaluated. If it's value it true then the whole expression is evaluated as <value-if-true>. Otherwise, the whole expression is evaluated as <value-if-false>.
In the code itself, for a, n % 2 is the boolean expression - js divides n by 2 and takes the remainder. The remainder is either 1 or 0. js implicitly converts those to true and false respectively. So if n is odd we get
a = 0
and if it's even we get
a = i
as the algorithm requires.
This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 6 years ago.
I am seeking help understanding why the way I am using anonymous functions are erroring in some circumstances.
In the snippet below I popuplate 2 arrays with functions which are invoked at a later stage.
var arr1 = [];
var arr2 = [];
// callback function
var func = function(i){
return function(){$("body").append(i);}
};
var i = 1;
while(i <= 5)
{
arr1.push(function(){$("body").append(i);});
arr2.push(func(i));
i++;
}
// invoke functions
$("body").append("Output from arr1 == ");
for(var c = 0; c < arr1.length; c++){ arr1[c](); }
$("body").append("<br> Output from arr2 == ");
for(var c = 0; c < arr1.length; c++){ arr2[c](); }
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
Now, I think I understand why arr1 outputs 66666, because at the time of invocation, i == 6 and becuase it has not been stored as a param within that function at time of creation, i retains it's last known value?
What I really don't understand is why i recieve TypeError: arr2[c] is not a function When I change the callback function to:
var func = function(i){
$("body").append(i);
};
Why does this happen and is this the most appropriate / elegant way to achieve this functionality.
For the first part of the question, you are right, it will retain the latest known value. To avoid that, you need to use closure. For example:
(function(x) {
arr2.push(func(x));
})(i);
Closure can often be used as a "scope hack". In this case, it will only ensure that the injected value is the constant current value of i.
For the second part of the question, your confusion is probably caused by the fact that you insert the result of the function (i.e.: func(i)) instead of the function itself (func).
In the first scenario, you return an explicit function(){...} so your array will contain a function that you can call with operator "()".
In the second scenario, in which you return $(<...>).append(), it's not guaranteed anymore that your return value is a function. In that case, the result is "jQuery" (see: jQuery append) and not a function, hence the error is correct. You cannot use the value as a function.
One way out is:
arr2.push(func);
Then:
arr2[c](c);
Or probably this:
(function(x){
arr2[x](x);
})(c);
If needed, it can be helpful to always console.log what is populated inside your arrays before assuming it right. Since Javascript is not a typed language, you may go for a little while before suspecting anything wrong as it will seem work as intended.
Hope this helps!
This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 1 year ago.
I've never had to use callback functions before, so I may have made a completely stupid mistake. I think I somewhat understand the problem here, but not how to solve it.
My code (a bit simplified) is:
for (var i = 0; i < some_array.length; i++) {
var title = some_array[i];
$.getJSON('some.url/' + title, function(data) {
do_something_with_data(data, i);
}
Now as far as I understand, this anonymous function will only be called if getJSON() has received the data. But by this point, i does not have the value I would require. Or, as far as my observation goes, it has the last value it would have after the loop is done (shouldn't it be out of bounds?).
As a result, if the array had a size of 6, do_something_with_data() would be called five times with the value 5.
Now I thought, just pass i to the anonymous function
function(data, i) { }
but this does not seem to be possible. i is undefined now.
You need to understand what a closure is. In JavaScript, there are certain rules about the scope of each variable.
The scope for variables declared implicitly or with var is the nearest/current function (including "arrow functions"), or if not in a function, then the window or other global object appropriate for the execution context (e.g., in Node, global).
The scope for variables declared with let or const (in ES5 and up) is the nearest statement block { /* not an object, but any place that will take executable statements here */ }.
If any code can access a variable in the current scope or in any parent scope, this creates a closure around that variable, keeping the variable live and keeping any object referred to by the variable instantiated, so that these parent or inner functions or blocks can continue to refer to the variable and access the value.
Because the original variable is still active, if you later change the value of that variable anywhere in the code, then when code with a closure over that variable runs later it will have the updated/changed value, not the value when the function or scope was first created.
Now, before we address making the closure work right, note that declaring the title variable without let or const repeatedly in the loop doesn't work. var variables are hoisted into the nearest function's scope, and variables assigned without var that don't refer to any function scope get implicitly attached to the global scope, which is window in a browser. Before const and let existed, for loops in JavaScript had no scope, therefore variables declared within them are actually declared only once despite seeming to be (re)declared inside the loop. Declaring the variable outside the loop should help clarify for you why your code isn't working as you'd expect.
As is, when the callbacks run, because they have a closure over the same variable i, they are all affected when i increments and they will all use the current value of i when they run (which will as you discovered be incorrect, because the callbacks all run after the loop has completely finished creating them). Asynchronous code (such as the JSON call response) does not and cannot run until all synchronous code finishes executing--so the loop is guaranteed to complete before any callback is ever executed.
To get around this you need a new function to run that has its own scope so that in the callbacks declared inside of the loop, there is a new closure over each different value. You could do that with a separate function, or just use an invoked anonymous function in the callback parameter. Here's an example:
var title, i;
for (i = 0; i < some_array.length; i += 1) {
title = some_array[i];
$.getJSON(
'some.url/' + title,
(function(thisi) {
return function(data) {
do_something_with_data(data, thisi);
// Break the closure over `i` via the parameter `thisi`,
// which will hold the correct value from *invocation* time.
};
}(i)) // calling the function with the current value
);
}
For clarity I'll break it out into a separate function so you can see what's going on:
function createCallback(item) {
return function(data) {
do_something_with_data(data, item);
// This reference to the `item` parameter does create a closure on it.
// However, its scope means that no caller function can change its value.
// Thus, since we don't change `item` anywhere inside `createCallback`, it
// will have the value as it was at the time the createCallback function
// was invoked.
};
}
var title, i, l = some_array.length;
for (i = 0; i < l; i += 1) {
title = some_array[i];
$.getJSON('some.url/' + title, createCallback(i));
// Note how this parameter is not a *reference* to the createCallback function,
// but the *value that invoking createCallback() returns*, which is a function taking one `data` parameter.
}
Note: since your array apparently only has titles in it, you could consider using the title variable instead of i which requires you to go back to some_array. But either way works, you know what you want.
One potentially useful way to think about this that the callback-creating function (either the anonymous one or the createCallback one) in essence converts the value of the i variable into separate thisi variables, via each time introducing a new function with its own scope. Perhaps it could be said that "parameters break values out of closures".
Just be careful: this technique will not work on objects without copying them, since objects are reference types. Merely passing them as parameters will not yield something that cannot be changed after the fact. You can duplicate a street address all you like, but this doesn't create a new house. You must build a new house if you want an address that leads to something different.
You could create a closure using an immediate function (one that executes right away) that returns another function:
for (var i = 0; i < some_array.length; i++) {
var title = some_array[i];
$.getJSON('some.url/' + title, (function() {
var ii = i;
return function(data) {
do_something_with_data(data, ii);
};
})());
}
If you can modify the service at some.url, it would be much better if rather than making a separate HTTP request for each item in some_array, you simply passed every item in the array in a single HTTP request.
$.getJSON('some.url', { items: some_array }, callback);
Your array will be JSON serialized and POSTed to the server. Assuming some_array is an array of strings, the request will look like this:
POST some.url HTTP/1.1
...
{'items':['a','b','c', ... ]}
Your server script should then deserialize the JSON request from the request body and loop over each item in the items array, returning a JSON-serialized array of responses.
HTTP/1.1 200 OK
...
{'items':[{id:0, ... }, {id:1, ... }, ... ]}
(Or whatever data it is you're returning.) If your response items are in the same order as the request items, it is easy to piece things back together. In your success callback, simply match the item index with some_array's index. Putting it all together:
$.getJSON('some.url', { items: some_array }, function(data) {
for (var i = 0; i < data.items.length; i++) {
do_something_with_data(data.items[i], i);
}
});
By 'batching up' your requests into a single HTTP request like this, you'll significantly improve performance. Consider that if each network round-trip takes at least 200ms, with 5 items, you're looking at a minimum 1 second delay. By requesting them all at once, network delay stays a constant 200ms. (Obviously with larger requests, server script execution and network transfer times will come in to play, but performance will still be an order of a magnitude better than if you issue a separate HTTP request for each item.)
Create N closures and pass in the value of 'i' each time, like so:
var i, title;
for (i = 0; i < some_array.length; i++) {
title = some_array[i];
$.getJSON('some.url/' + title, (function(i_copy) {
return function(data) {
do_something_with_data(data, i_copy);
};
})(i));
}
I think some browsers have trouble with making multiple asynchronous calls at the same time, so you could make them one at a time:
var i;
function DoOne(data)
{
if (i >= 0)
do_something_with_data(data, i);
if (++i >= some_array.length)
return;
var title = some_array[i];
$.getJSON('some.url/' + title, DoOne);
}
// to start the chain:
i = -1;
DoOne(null);
I had exactly the same issue as the OP but solved it a different way. I replaced my JavaScript 'for' loop with a jQuery $.each which for each iteration calls a function which I think gets over the callback 'timing' issue. And I combined my external data arrays into a JavaScript object so that I could reference both the parameter I was passing on the JSON URL and the other field in the same element of that object. My object elements came out of a mySQL database table using PHP.
var persons = [
{ Location: 'MK6', Bio: 'System administrator' },
{ Location: 'LU4', Bio: 'Project officer' },
{ Location: 'B37', Bio: 'Renewable energy hardware installer' },
{ Location: 'S23', Bio: 'Associate lecturer and first hardware triallist' },
{ Location: 'EH12', Bio: 'Associate lecturer with a solar PV installation' }
];
function initMap() {
var map = new google.maps.Map(document.getElementById('map_canvas'), {
center: startLatLon,
minZoom: 5,
maxZoom: 11,
zoom: 5
});
$.each(persons, function(x, person) {
$.getJSON('http://maps.googleapis.com/maps/api/geocode/json?address=' + person.Location, null, function (data) {
var p = data.results[0].geometry.location;
var latlng = new google.maps.LatLng(p.lat, p.lng);
var image = 'images/solarenergy.png';
var marker = new google.maps.Marker({
position: latlng,
map: map,
icon: image,
title: person.Bio
});
google.maps.event.addListener(marker, "click", function (e) {
document.getElementById('info').value = person.Bio;
});
});
});
}