How to loop through this data: (I have no control over format)
{"rowCount":3,"1":{"K":"2009","V":"Some Data"},"2":{"K":"2010","V":"More Data"}}
Above is a console.log(results) and results is a string
var r = JSON.parse(results);
var t;
for(var i=1;i<=r.rowCount;i++) {
t=r[i].V;
tableData.push(
{title:t, year:'2009', hasChild:true, color: '#000'}
);
}
Error: TypeError: 'undefined' is not an object (evaluating 'r[i].V')
I cannot get it to evaluate the variable i. What am I doing wrong?
Thanks
UPDATE
The incoming data had a bad rowcount causing the error. The accepted answer however is correct... just user error on my part not catching the bad incoming data. Had I put a console.log inside the loop I would have realized the error was actually happening after two successful loops. oops
I assume r.rowCount should be j.rowCount.
Ideally you should also initialise the i variable if you haven't already (i.e. with the var keyword).
(I've also moved the var t declaration outside the loop, to make it clear that it's the same t throughout and you're just changing its value. You shouldn't redeclare it with var each time – although I doubt this affects the output.)
var j = {"rowCount":2,"1":{"K":"name","V":"john"},"2":{"K":"name","V":"sue"}};
var t;
for (var i = 1; i <= j.rowCount; i++) {
t = j[i].V;
console.log(t);
}
Working demo – JSFiddle
I'm trying to use console.log to get the value of the self.subcategories object after it is created in the subscribe function below. I always get c() in my console from the console.log(self.subcategories) below. I know that the data is there, as I can use it in a different piece of code. I just want to be able to see it in console.log so I can get more info and know what to do with it.
function QuestionFilter(data, categories, getSubcategoriesByCategoryUrl, getQuestionsBySubcategoryUrl) {
var self = this;
self.categories = ko.observableArray(categories);
self.subcategories = ko.observableArray([]);
self.selectedCategory = ko.observable();
self.selectedCategory.subscribe(function(category) {
function search(nameKey, myArray){
for (var i=0; i < myArray.length; i++) {
if (myArray[i].parentCategory_id === nameKey) {
self.subcategories.push(myArray[i]);
}
}
}
search(category, categories);
console.log(self.subcategories);
});
};
The Knockout.js library is included and so there are a couple of references to it. As I mentioned, everything works, I just want to be able to log the object to help me write more code. Any ideas?
Have a look at the documentation (emphasis mine):
Reading information from an observableArray
Behind the scenes, an observableArray is actually an observable whose value is an array (plus, observableArray adds some additional features described below). So, you can get the underlying JavaScript array by invoking the observableArray as a function with no parameters, just like any other observable. Then you can read information from that underlying array. For example,
alert('The length of the array is ' + myObservableArray().length);
alert('The first element is ' + myObservableArray()[0]);
I have read this answer but still have a headache on making the code work.
I have a slightly different needs. Instead of alerting I have to bind each object.
My code is:
for (var i = 0; i < markers_length; i++) {
events_number = data.markers[i].events_number //data.markers is a multidimentional array
marker = L.marker([ data.markers[i].latitude , data.markers[i].longitude ]); //just create the new object
marker.on('mouseover', function(){
return function(){
this.bindPopup(" Found"+events_number+" event(s)").openPopup();
}
}(i) );
}
I am using leaflet if you ask. For a single object the bindPopup would work like:
marker.on('mouseover', this.bindPopup('hi').openPopup());
The trouble is that the above code gives the last object for all the. I assume that there is a problem with the this and the level of the functions. So how can I bind each marker with a separate text?
it should be:
marker.on('mouseover', function(en){
return function(){
this.bindPopup(" Found"+en+" event(s)").openPopup();
}
}(events_number) );
You have to pass the value that you want to be saved in the closure (events_number in this case), and the function has to take a parameter to receive that value and use it in the closure (en in my code).
As part of the web app I'm building, multiple commands can come in from the server at once.
When these commands are processed, data will tend to be updated and then fairly heavy page HTML generation can occur.
At the moment when multiple similar commands come in, the program will process the data, then parts of the page are regenerated each time. This can lead to a lot of wasted processing time.
As such, I'm trying to make a callstack, so when a command is processed it looks to see if the function that the command triggers is in the stack and if it isn't found it adds it.
My problem is with keeping the called function from the callstack in the correct scope while also being able to eliminate duplicates.
Base Code:
var tools = {
functions:{
function1: function(){
return this;
},
function2: function(){
}
},
unique: function(arr){
var returnArr=[],
x, i;
for (x=0; x<arr.length; x++){
for (i=(x+1); i<arr.length; i++){
if (arr[x]===arr[i]){
arr.splice(i, 1);
}
}
returnArr.push(arr[x]);
}
return returnArr;
}
}
Example 1:
var callstack=[tools.functions.function1, tools.functions.function1];
callstack = tools.unique(callstack);
for (var x=0; x<callstack.length; x++){
console.log(callstack[x](), "should equal tools.functions");
}
This fails as "this" will return [function()]
Example 2:
var callstack=[
(function(){return tools.functions.function1()}),
(function(){return tools.functions.function1()})
];
callstack = tools.unique(callstack);
for (var x=0; x<callstack.length; x++){
console.log(callstack[x](), "should equal tools.functions");
}
This fails because you can't ensure that the functions are unique, so it'll still run the function twice.
This can be worked around by using two arrays (one that keeps track of the names of the functions, one that holds the encapsulated functions) and keeping them in sync, but I can't help but feel there must be a cleaner way using .call but I can't see it.
JSFiddle: http://jsfiddle.net/Kj6E8/
Generically speaking it is impossible to preserve the value of this in the way that you require. As soon as you push those functions into the array, the object that those functions once resided on is lost.
If you are only calling functions on a single object, you need to bind your functions to that object in some way. There are many ways of doing that, for example, you could pass the object to bind those function to in your unique function.
http://jsfiddle.net/8WAZb/1/
var tools = {
functions:{
function1: function(){
return this;
},
function2: function(){
}
},
unique: function(arr, o){
var returnArr=[],
x, i;
for (x=0; x<arr.length; x++){
for (i=(x+1); i<arr.length; i++){
if (arr[x]===arr[i]){
arr.splice(i, 1);
}
}
returnArr.push(this.bind(arr[x], o));
}
return returnArr;
},
bind: function(f, o) {
return function () {
return f.apply(o, arguments);
}
}
}
console.info("Example 1:");
var callstack=[tools.functions.function1, tools.functions.function1];
callstack = tools.unique(callstack, tools.functions);
console.log("Callstack:",callstack);
for (var x=0; x<callstack.length; x++){
console.log(callstack[x](), "should equal tools.functions");
}
How and when you infer the value of this really depends on when you have that object in scope. Presumably you are more likely to have that object in scope during the callstack creation phase, and perhaps not necessarily at the point where you wish to invoke the functions in the callstack. If that is the case, binding as early as possible (i.e. during the reduction to a unique callstack as demonstrated) seems like a sensible option.
In JavaScript, this is defined from the outside caller, so when you do:
callstack[x]()
You are bringing the window scope into the functions. If you want to bring in the tools.functions object, you need to do:
callstack[x].call(tools.functions);
http://jsfiddle.net/Yv4sM/
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;
});
});
});
}