I'm trying to learn JavaScript and I saw a code to change the css style of a web page depending on the button you press.
I can't understand why or how a for loop indicate witch button was press. Here is the javascript part:
var buttons = document.getElementsByTagName("button");
var len = buttons.length;
for (i = 0; i < len; i++) {
buttons[i].onclick = function() {
var className = this.innerHTML.toLowerCase();
document.body.className = className;
};
}
http://jsfiddle.net/qp9jwwq6/
I looked on the net and w3 school but they don't explain that code with a for loop. Can someone explain it to me?
Thank you
Lets break it down.
First we need to have access to the DOM element on the page, so we do that by using a method on the document itself which will return the element we want to manipulate.
var buttons = document.getElementsByTagName("button");
The buttons var will be a list of ALL the buttons on the page. We want to do something with all of them, so first we cache the length of the list, i.e, count how many buttons we have.
var len = buttons.length;
Then we basically say: set i to 0, and step it up one until its equal to the number of buttons we have.
for (i = 0; i < len; i++) {
Now, to access one button from the list, we need to use the brackets notation. So buttons[0] is the first element, buttons[1] is the second, etc. Since i starts at 0, we put i in the brackets so that on each iteration it will access the next button in the list.
buttons[i].onclick = function() {
var className = this.innerHTML.toLowerCase();
document.body.className = className;
};
}
This is equivalent of doing:
buttons[0].onclick = function() {
var className = this.innerHTML.toLowerCase();
document.body.className = className;
};
buttons[1].onclick = function() {
var className = this.innerHTML.toLowerCase();
document.body.className = className;
};
buttons[2].onclick = function() {
var className = this.innerHTML.toLowerCase();
document.body.className = className;
};
// etc.
But of course that is super inefficient, and we may not know how many buttons the page has. So we get all the buttons there, find out how many there are, then go through each button and assign an event handler to it along with a new class.
Now, looking at the onclick handler itself we can see that it first finds the HTML within the button being clicked, turns it into lowercase letters, and assigns it to a variable:
var className = this.innerHTML.toLowerCase();
By using this we're ensuring that each button will know to get it's own innerHTML when clicked. We're not tracking which button is which, we're just telling each button to check it's own content.
Then what it does is change the class of the body HTML element to whatever it is that we just parsed
document.body.className = className;
So say you have something like
<button>success</button>
<button>failure</button>
<button>warning</button>
Clicking the first button would set the <body> element's class to success, clicking the second would set it to failure, and the third would set it to warning.
First line saves all buttons in a variable called buttons. This is actually an array since there can be several buttons on the page. Then you iterate through each button and define a function which should be executed onclick. Lets say you have 2 buttons then it will be buttons[0] and buttons[1] which get the function.
Firstly, speaking generally, the underlying basis for this code is a little wonky and unusual and non-robust, so don't anticipate that you're on the brink of learning any powerful insight into JavaScript or code design.
On to the answer:
The for-loop does not "indicate" which button was pressed. Rather, it loops through every button element on the page and assigns the exact same function definition to the onclick attribute of each element. The code that ends up running when a particular button element is clicked (here I'm talking about the function body) assigns a CSS class to the body element by assigning to document.body.className.
Your question is asking how the function knows which class name to assign to document.body.className. The function grabs the class name from the innerHTML of the button element, which is accessible as this.innerHTML (because in an event handler, this is a reference to the element on which the triggering event occurred). The HTML <button> element is a little bit special, in that, although it is generally a simple-looking button, it is also a non-leaf node, meaning it contains its own HTML. You can put a lot of things in there, but in this example, they just have a plain text node which consists of exactly (or nearly exactly) the class name (Normal for one and Changed for the other). That's how the function can get a CSS class name that is specific to that button; it grabs it from the text inside the clicked <button> element.
I said "nearly exactly" back there because there's actually a letter-case difference between the button text and the actual CSS classes they've defined in the CSS rules (which are normal and changed). That's why they have to lower the letter-case of the extracted button text (toLowerCase()) before assigning the class name. CSS classes are case-sensitive (see Are CSS selectors case-sensitive?).
As I said, this is unusual code. It is rather inadvisable to create a mapping (especially an inexact mapping!) between plain HTML text and code metadata (CSS classes in this case).
Related
Hi I have been writing a Javascript quiz whilst learning Javascript, but have encountered a problem.
I have one function that dynamically creates the question/answers with radio buttons to mark off the questions.
When I use this second function to attempt to remove the question/answers so I can show the new ones; it removes the text (in p tags) but doesn't remove the radio buttons, even though they also show as children to the form element.
function removeLastQuestions() {
var allQuestions = document.getElementById('questionForm');
for (var i = 0; i < allQuestions.children.length; i++) {
allQuestions.removeChild(allQuestions.children[i]);
}
}
The question/answers and buttons are contained within a form with the id of "questionForm"
I guess I could put the whole form within a div and remove the form, but I'm wondering why looping over them isn't working. I'm trying to do it without using Jquery.
Thanks for any help.
Try this way:
var node = document.getElementById('questionForm');
while (node.hasChildNodes()) {
node.removeChild(node.lastChild);
}
This will remove all form elements.
JSFiddle demo
I am trying to setup an interchange using two texts boxes with a command button in between.
The idea is you type a reference/code in the left hand text box, click the button and it generates an alternative reference/code in the right hand text box.
The point being the user can check alternate bearing references if they can't find what they are looking for with the one they have.
The code I use so far is:
<script type="text/javascript">
oldRef = new Array ("Z582","T608","A173");
newRef = new Array ("C850","S708","X449");
function convert()
{
document.getElementById("v2").value = "";
for (index=0 ; index < oldRef.length ; index++)
{
if ( document.getElementById("v1").value == oldRef[index] )
document.getElementById("v2").value = newRef[index];
}
}
</script>
V1 and V2 refer the the text box ID.
This works with the text boxes but I don't know how to incorporate the command button into this so that they need to click the button in the middle for it to generate.
Any suggestions would be much appreciated.
Best
Will
Its pretty Easy stuff what you need to do is to use onclick of the button like this
document.getElementById('button').onclick = function () {
document.getElementById("v2").value = "";
for (var index=0 ; index < oldRef.length ; index++) {
if (document.getElementById("v1").value == oldRef[index])
document.getElementById("v2").value = newRef[index];
}
}
Here is a demo
I hope this is waht you want....
So essentially there are two things that you need to accomplish what you asked. You need to create a element within your HTML, in this case a button. You then need to catch the event that you want to catch from that element and then execute you convert function.
This is one example of accomplishing this:
So create an button within your HTML
<button id="btn_command">Command</button>
Then in Javascript you want to target that button and add an event listener to that button. In the example the below the variable var btnCommand is set to the html button by using the getElementById method to get that button with that id. Then we add and event listener to that element that when clicked it executes your convert function.
var btnCommand = document.getElementById ("btn_command") ;
btnCommand.addEventListener("click", convert, false) ;
If you want to use jQuery you would do something like this.
$('#btn_command').on('click', function() { convert(); });
Here is another quick and dirty way to just test you button with your function. It is not a best idea to mix your javascript inline with your html but just to test your button and if your convert function is doing that you think you could just say
<button onClick="convert()">Command</button>
Well there are few ways to accomplish what you asked. Happy Coding!
I have a chrome extension written in pure JS which is basically composed of functions which append data to links on the page.
An example of the functions which basically just appends a click integer to the link element passed in from another function:
function addCounttoLinks(link, counts) {
var clicks = counts.split(":")[1].split(",")[0].trim();
var count = document.createTextNode((" (" + clicks + ")"));
var dspan = document.createElement("span");
dspan.appendChild(count);
dspan.style.fontSize = "10px"; dspan.style.textAlign = "center";
link.appendChild(dspan);
}
What is the most efficient way to execute/unexecute these functions, so that I can implement a "on/off" button for the user to click which would add/remove the data appended by the extension. Is the only solution to write functions to remove the previously created elements?
Put a class on the span (say, my-nifty-extension-extra — you want it to be fairly unlikely to clash with something else), then add a class to body (say, hide-my-nifty-extension) when hiding the data, and remove it when showing it. Then use this style rule:
body.hide-my-nifty-extension span.my-nifty-extension-extra {
display: none;
}
E.g., just before your appendChild:
dspan.className = "my-nifty-extension-extra";
And then:
function showExtraData() {
document.body.classList.remove('hide-my-nifty-extension');
}
function hideExtraData() {
document.body.classList.add('hide-my-nifty-extension');
}
(One of the joys of writing a Chrome extension is that you know you have things like classList.)
I have dynamically created span elements that listen to an onkeyup action.
For example: When a user types into a textbox, it should replace the text inside the <span> too.
lets say I create N elements of span.
N-1 of then works perfectly, but the last one does not.
When I use Inspect Element (Chrome browser) and make the call
document.getElementById("span...").innerHTML
I see the new text, but on screen I still see the old one.
My code:
1. This is the part when i dynamically create the text inputs
titleInput.onkeyup = (function ()
{
var inputObject = titleInput;
var chartIndex = numberOfTds; return function () {
UpdateTitleInMagnifiedView(inputObject, chartIndex);
} })();
UpdateTitleInMagnifiedView - function that updates the span elemnts
document.getElementById("title_magnified_" + chartIndex).innerHTML = inputObject.value;
I have have multiple divs' with similar code within it, but also has a unique id within the main div that calls a toggleClass & slideToggle function. I'm trying to create a loop so that I do not have to write similar code for each element.
--- working code --- (where numeric value after the id would change)
$('#er1').click(function() {
$('#er1').toggleClass('but-extra-on');
$('#cr1').toggleClass('but-closerow-on');
$('#er1').next('div').slideToggle('slow');
return false;
});
-- not working code -- (I want to have functions for the click of #er1, #er2, #er3 etc.)
var count = 1;
while (count < 10){
var curER = 'er'+count;
var curCR = 'cr'+count;
$('#'+curER).click(function() {
$('#'+curER).toggleClass('but-extra-on');
$('#'+curCR).toggleClass('but-closerow-on');
$('#'+curER).next('div').slideToggle('slow');
});
count++;
}
* for some reason, when I use the while code, whenever I click #er1, #er2, #er3 etc.. only the event for #er9 toggles.
You can solve this problem, by using the $(this) selector for the one that you are clicking, and attaching an html data attribute to the div, specifying the other div that you want to change when you click it and selecting the other one with that... make sense.. probably not? Check out Solution 1 below.
The second solution is to use jQuery Event Data to pass the count variable into the event listener.
Solution 1: http://jsfiddle.net/Es4QW/8/ (this bloats your html a bit)
Solution 2: http://jsfiddle.net/CoryDanielson/Es4QW/23/
I believe the second solution is slightly more efficient, because you have slightly less HTML code, and the $(this) object is slightly smaller. Creating the map and passing it as event data, I believe, is less intensive... but... realistically... there's no difference... the second solution has cleaner HTML code, so you should use that.
Solution 3 w/ .slideToggle(): http://jsfiddle.net/CoryDanielson/Es4QW/24/
Edit: Updated solutions by passing in the selected elements instead of the selectors. Now each click event will not do a DOM lookup as it did before.
I've run into this problem before. I fixed it by extracting the event listener code into its own function like so:
for (var i = 1; i < 10; i++) {
attachClickEvent('er'+i, 'cr'+i);
)
function attachClickEvent(cr, er)
{
$('#'+er).click(function() {
$(this).toggleClass('but-extra-on');
$('#'+cr).toggleClass('but-closerow-on');
$(this).next('div').slideToggle('slow');
});
}