I'm a beginner in jquery and ajax. While i was going through some example online, i came across the following piece of code and wondered what exactly it does.
lines = newLine.split('#');
jQuery.each(lines, function(lineNo, line) {
eval("linedata = " + line);
data.push(linedata);
});
I'm not a programmer, but just trying to understand its functionality. Can anyone help me?
The each function iterates over an array which is supplied as the first parameter. During each iteration the index and element are passed into a function that is performed. The function is passed as the second parameter to the each function.
Read more on the jQuery Documentation
In the example you have provided a string newLine is split into an array using # as the delimiter.
The each function then iterates over the newly created array, assigning the value of each element to a variable linedata and pushes linedata onto another array.
This could be more easily achieved with the following, since the call to eval is unnecessary:
jQuery.each(lines, function(lineNo, line) {
data.push(line);
});
I pretended, for a moment, that I was a new programmer. This is how you should go about looking into things from here on out:
1.) Ok, I don't know what this first line is doing. It's splitting something (based on the split word). Hmmm let's Google for "split javascript". This is the first thing that comes up. From here, you may be wondering what a String is, so you would search for that as well).
2.) Ok so now I know that splitting a String gives me an array (again you probably looked this up by this step) of the newLine substrings that were separated by the # character. Cool. So let's look into what jQuery.each does. I google "jQuery.each" and this is the first thing that comes up.
Awesome! Now you understand what a String is, an Array, the split function from String as well as what jQuery.each is. :D
EDIT: As you move forward, you'll realize that W3C is generally an inferior source of information. I simply linked to it since it was literally the first thing that came up when I Googled "split javascript". Overall it does the job for giving you a good overview of certain things when you're learning them for the first time.
Related
I have a very specific problem concerning a regular expression matching in Javascript. I'm trying to match a piece of source code, more specifically a portion here:
<TD WIDTH=100% ALIGN=right>World Boards | Olympa - Trade | <b>Bump when Yasir...</b></TD>
The part I'm trying to match is boardid=106121">Olympa - Trade</a>, the part I actually need is "Olympa". So I use the following line of JS code to get a match and have "Olympa" returned:
var world = document.documentElement.innerHTML.match('/boardid=[0-9]+">([A-Z][a-z]+)( - Trade){0,1}<\/a>/i')[1];
the ( - Trade) part is optional in my problem, hence the {0,1} in the regex.
There's also no easier way to narrow down the code by e.g. getElementsByTagName, so searching the complete source code is my only option.
Now here's the funny thing. I have used two online regex matchers (of which one was for JS-regex specifically) to test my regex against the complete source code. Both times, it had a match and returned "Olympa" exactly as it should have. However, when I have Chrome include the script on the actual page, it gives the following error:
Error in event handler for 'undefined': Cannot read property '1' of null TypeError: Cannot read property '1' of null
Obviously, the first part of my line returns "null" because it does not find a match, and taking [1] of "null" doesn't work.
I figured I might not be doing the match on the source code, but when I let the script output document.documentElement.innerHTML to the console, it outputs the complete source code.
I see no reason why this regex fails, so I must be overlooking something very silly. Does anyone else see the problem?
All help appreciated,
Kenneth
You're putting your regular expression inside a string. It should not be inside a string.
var world = document.documentElement.innerHTML.match(/boardid=[0-9]+">([A-Z][a-z]+)( - Trade){0,1}<\/a>/i)[1];
Another thing — it appears you have a document object, in which case all this HTML is already parsed for you, and you can take advantage of that instead of reinventing a fragile wheel.
var element = document.querySelector('a[href*="boardid="]');
var world = element.textContent;
(This assumes that you don't need <=IE8 support. If you do, there remains a better way, though.)
(P.S. ? is shorthand for {0,1}.)
I'm really stuck on this javascript question!
So I'm making a web page that will be completely animated (so it can be used for display for example in a television). That animation will be configurable by the user (stored in a database).
Right now, I've already made the code to store the configuration and to get the configuration (I do an AJAX call and save the configuration in an array of json objects) and everything is as it should be.
The problem is in the animation in which I go through the array and use setTimeout function to create animations. To iterate through the array I rotate it
(I use array.push(array.shift()) according to the answer here).
The first time the intervalmaster function is used, everything goes according to plan, but when the function is called again I need to rotate the array once more (because of the last animation) and the array just doesn't rotate!
Bellow I've left a portion of the code that I'm using that reproduces the problem I'm getting. I've also added the array jsonanima with some possible values (In reality the array is probably much bigger and with higher values).
I really don't understand what is happening, I've also considered that this could be a problem of the multiple setTimeout functions because I've read somewhere (couldn't find the link, sorry!) that is not really advised to use multiple setTimeout.
If that's the case is there any other way to do this?
Thank you in advance!
EDIT: Thanks to the comment from mplungjan I've realized that if change the console.log(jsonanimate) to console.log(JSON.stringfy(jsonanima)) it outputs the correct values (the json array rotated). This got me even more confused! Why do I need to JSON.stringfy to get the array in the correct order?!
Anyway, can't test this with the full code now as I'm not in the office, tomorrow I'll give more feedback. Thank you mplungjan.
EDIT2: Finally solved my problem! So the thing was the call to the function recursivegroup (recursivegroup(0);), this call was made before I rotated the array, so when restarting the animation the array would still have the incorrect values and every sub-sequential value was wrong.
A special thanks to mplungjan and trincot for the comments that helped me debug this problem.
Bellow I leave the code corrected so anybody with the same problem can check it out.
jsonanima=[{"VD":5,"A":10,"diff":0.25},{"L":7,"IE":8,"diff":0.25}];
function intervalmaster(flag){
function recursivegroup(index)
{
if(index==0)
{
//animateeach(jsonanima,0);
}
setTimeout(function(){
//HERE IT WORKS
jsonanima.push(jsonanima.shift());
console.log(JSON.stringify(jsonanima));
//animateeach(jsonanima,0);
//removed the if statement, since it was irrelevant as mplungjan noted
recursivegroup(index+1);
},(jsonanima[0]['diff'])*60*1000);
}
//Changed this
//recursivegroup(0);
var mastertime=0;
for(var key in jsonanima)
{
mastertime+=(jsonanima[key]['diff']);
}
console.log(mastertime,flag);
console.log(JSON.stringify(jsonanima));
if(flag==true)
{
jsonanima.push(jsonanima.shift());
console.log(JSON.stringify(jsonanima));
}
//changed to here
recursivegroup(0);
masterinterval=setTimeout(function(){intervalmaster(true)},mastertime*60*1000);
}
intervalmaster(false);
I am sure there is something simple that I am missing but I am stumped here.
The issue is that I am looping through an array of strings and using the string value to search for a part of that string using indexOf. The first time around the loop the index of is finding what I am looking for but the second time it is not.
Here is a fiddle - http://jsfiddle.net/jeremywrags/uSwjG/1/
the line that seems to be not working is this
var aliasIndex = fromclause.indexOf(" " + tableAlias + " " );
I am trying to build a SQL parser for a cloud app and the use case here is that when a table is aliased I need to get the original table name so that I can look up the table columns. The first time around the loop index of returns the index and then the table name. The second time around the index of is -1 and the table name is not retrieved.
If I need to provide more context please let me know.
thanks
It's not matching because on the second pass, tableAlias is the string " b" (note the space). So then you search for " b " (note two leading spaces), which isn't there.
Rather than using alert, use the debugger built into your browser. You can set breakpoints in the code, step through line by line, inspect variables, etc., etc. Doing that with this would have shown you, when looking at the variable tableAlias, that it had a leading space, hopefully helping you find the solution.
Here's what that looks like in Chrome's debugger, for instance:
(If you look at the jsFiddle source above the actual debugger's version, you'll see a debugger; statement in the code — normally you don't need that statement, you can just open your page, use the "Sources" tab to find your JavaScript file, navigate to the line, and click the margin to the left of it to set a breakpoint. But sometimes [for instance, when using jsFiddle], the debugger; statement is handy. What it does is, if the debugger is open, halts execution of the code at that point like a breakpoint does.)
I'm making a library, and I often inspect the result of Closure Compiler's output to see how it's doing things (I do have unit tests, but I still like to see the compiled code for hints of how it could compress better).
So, I found this very weird piece of code, which I never seen before.
variable : {
some();
code()
}
Note: this is not an object literal! Also, there is no ? anywhere that would make it a ?: conditional.
That code is in a regular function block (an IIFE).
variable, in this case, is an undefined variable. There's no code making it true, false, or whatever, and just to make sure, I put a console.log in there and indeed, I get a ReferenceError.
Please do note that I test my code in IE8 too, so this isn't just in modern browsers. It seems to be standard, plain old javascript.
So let's experiment with it. Firing up Chrome's console, I get this:
undeclaredVariable:{console.log('does this get logged?')} // yes it does.
trueValue:{console.log('what about this?')} // same thing.
falseValue:{console.log('and this?')} // same thing.
but then...
(true):{console.log('does this work too?')} // SyntaxError: Unexpected token :
...and...
so?{console.log('is this a conditional?')}:{alert(123)} // Unexpected token .
So what does it do?
thisThing:{console.log('is used to declare a variable?')}
thisThing // ReferenceError: thisThing is not defined
Please, I'd love it if someone could explain to me what this code is meant to do, or at least what it does.
It is a label
Provides a statement with an identifier that you can refer to using a
break or continue statement.
For example, you can use a label to identify a loop, and then use the
break or continue statements to indicate whether a program should
interrupt the loop or continue its execution.
Another common place you see it is when people stick the wonderful and useless javascript: on event handlers.
This is a label (the bit ending with a colon) followed by a block (the code surrounded by the curly brackets).
Blocks usually follow control statements, like if(...) { /*block*/ }, but they can also simply stand on their own, as in your example.
Labels allow jumping up several loops at a time with a continue or break; see the linked MDN page for several examples, such as:
var itemsPassed = 0;
var i, j;
top:
for (i = 0; i < items.length; i++){
for (j = 0; j < tests.length; j++)
if (!tests[j].pass(items[i]))
continue top;
itemsPassed++;
}
Here, top: is a label that code inside the inner loop can jump to, in order to escape to the outer loop.
For the sake of anyone who doesn't know what JSON is, and sees a colon in what might actually be an object, and is trying to figure out what it is, and finds this discussion, a colon is also used in JSON. There is a practice of embedding functions in a JSON object. Which might be confusing (As it was to me) for anyone who happens to see this for the first time. (Everyone isn't born with the knowledge of JSON and JavaScript programmed into their brains.) So if you find yourself at this discussion, and you think that every time you see a colon in JavaScript, that it's a label, it might not be. It might be that it's a colon after a label, OR it might be part of JSON. In fact, a colon in JSON being shown as a string, is a lot more common than a label. JSON in the form of an object, will be displayed as [object Object], with all the content hidden. So, unless the JSON is in the form of a string, and you display an object to the console (console.log(object)) all you will see is [object Object]. It is common practice to write JavaScript code, wrapped in an object. In that case you will see the JSON in the form of code. That's when you'll ask yourself, "What is this? and what is that colon for?" Then you'll find yourself at this discussion, and be told that it's a label, when it's really part of JSON. The topic of this discussion is worded: "Please explain this usage of a colon in javascript", and then the "correct answer" is marked as having to do with a label. The correct answer is that a colon can be used in more than one way. So, if you don't know what JSON is, or think you know (like I did, but didn't really understand) read about it here:
JSON.org
That is just a label.
you can use continue [label name] (or break) in a loop to go to a label.
More explanations of what they are can be seen throughout the interwebs.
it is used for labeling an statement in jsvascript.check more detail here.
the labeled statement can be used with break and continue later.
The page having problems is...
http://schnell.dreamhosters.com/index.php?page=gallery#
I use Firebug to debug my jQuery and other code tidbits and it's been proving very useful for Javascript/jQuery debugging. However, at the same time, it's been one of the most frustrating debugging experiences I've ever gone through. I'm not sure why but sometimes it seems like I can copy someone else's methodology from a tutorial, character for character, and yet still come up with errors.
In any case, the problem here is that Firebug claims there is a bug on line 20 of the source.
missing : after property id
[Break on this error] $('#table').animate({"left: " + attr + "px"}, 2000);\n
This bug seems like a huge load to me because the colon is right there! And this is why debugging jQuery/Javascript is such a pain sometimes. The error messages are rather convoluted and sometimes don't even make a lick of sense to me. Or maybe that's just Firebug.
Either way, the goal I'm going for here is that I'm trying to dynamically change the animate function such that the more you click the left arrow, the further left the grid of images is shifted (due to the nature of the CSS 'left' property). I have Javascript variables and a hidden input tag to help hold essential values, but the major hurdle is getting the animate function to recognize these variables. Near as I can tell it will only accept string literals for arguments on how to animate and the documentation doesn't help me because it doesn't discuss the use of variables with animate, as if it's impossible.
Well, let's just say I don't like impossible, he likes to get in my way a lot.
The object literal passed to the animate function is not well formed, it should be:
$('#table').animate({left: attr + "px"}, 2000);
Edit: Looking closely to your code, you are also trying to get a value from an input with id = "count", and you have a missing # character to have an ID selector:
var count = +$('#count').val(); // get #count value as Number
You are also incrementing this count variable, but you should first convert it to Number, because the value attribute of input elements are string. (I did it using the unary plus operator on the right-hand side of the assignment).
You have to convert it to a number, because if you add two variables and one of them is a string, concatenation will take place:
"1" + 1 == "11"
Try:
$('#table').animate({left: attr}, 2000);
The "px" units of measurement here aren't necessary. That aside, the above is the correct creation of an anonymous object. You were just putting a string inside curly braces.