Top jquery frequently asked interview questions

Add table row in jQuery

What is the best method in jQuery to add an additional row to a table as the last row?

Is this acceptable?

$('#myTable').append('<tr><td>my data</td><td>more data</td></tr>');

Are there limitations to what you can add to a table like this (such as inputs, selects, number of rows)?

Source: (StackOverflow)

Is there an "exists" function for jQuery?

How can I check the existence of an element in jQuery?

The current code that I have is this:

if ($(selector).length>0) {
    // Do something
}

Is there is a more elegant way to approach this? Perhaps a plugin or a function?

Source: (StackOverflow)

Get selected text from a drop-down list (select box) using jQuery

How can I get a drop-down list selected text in jQuery, not using the selected value?

Source: (StackOverflow)

How can I make a page redirect using jQuery?

How can I redirect the user from one page to another using jQuery?

Source: (StackOverflow)

Checking if an element is hidden

In jQuery, it is possible to toggle the visibility of an element, using the functions .hide(), .show() or .toggle().

Using jQuery, how would you test if an element is visible or hidden?

Source: (StackOverflow)

Check checkbox checked property

I need to check the checked property of a checkbox and perform an action based on the checked property using jQuery.

For example, if the age checkbox is checked, then I need to show a textbox to enter age, else hide the textbox.

But the following code returns false by default:

if($('#isAgeSelected').attr('checked')) {
    $("#txtAge").show();
} else {
    $("#txtAge").hide();
}

How do I successfully query the checked property?

Source: (StackOverflow)

Checking a checkbox with jQuery?

I'd like to do something like this to tick a checkbox using jQuery:

$(".myCheckBox").checked(true);

or

$(".myCheckBox").selected(true);

Does such a thing exist?

Source: (StackOverflow)

$(this) selector and children?

I have a layout similar to this:

<div id="..."><img src="..."></div>

and would like to use a jQuery selector to select the child img inside the div on click.

To get the div, I've got this selector:

$(this)

How can I get the child img using a selector?

Source: (StackOverflow)

event.preventDefault() vs. return false

When I want to prevent other event handlers from executing after a certain event is fired, I can use one of two techniques. I'll use jQuery in the examples, but this applies to plain-JS as well:

1. `event.preventDefault()`

$('a').click(function (e) {
    // custom handling here
    e.preventDefault();
});

2. `return false`

$('a').click(function () {
    // custom handling here
    return false;
});

Is there any significant difference between those two methods of stopping event propagation?

For me, return false; is simpler, shorter and probably less error prone than executing a method. With the method, you have to remember about correct casing, parenthesis, etc.

Also, I have to define the first parameter in callback to be able to call the method. Perhaps, there are some reasons why I should avoid doing it like this and use preventDefault instead? What's the better way?

Source: (StackOverflow)

How can I upload files asynchronously?

I would like to upload a file asynchronously with jQuery. This is my HTML:

<span>File</span>
<input type="file" id="file" name="file" size="10"/>
<input id="uploadbutton" type="button" value="Upload"/>

And here my Jquery code:

$(document).ready(function () {
    $("#uploadbutton").click(function () {
        var filename = $("#file").val();

        $.ajax({
            type: "POST",
            url: "addFile.do",
            enctype: 'multipart/form-data',
            data: {
                file: filename
            },
            success: function () {
                alert("Data Uploaded: ");
            }
        });
    });
});

Instead of the file being uploaded, I am only getting the filename. What can I do to fix this problem?

Current Solution

I am using the jQuery Form Plugin to upload files.

Source: (StackOverflow)

jQuery document.createElement equivalent?

I'm refactoring some old JavaScript code and there's a lot of DOM manipulation going on.

var d = document;
var odv = d.createElement("div");
odv.style.display = "none";
this.OuterDiv = odv;

var t = d.createElement("table");
t.cellSpacing = 0;
t.className = "text";
odv.appendChild(t);

I would like to know if there is a better way to do this using jQuery. I've been experimenting with:

var odv = $.create("div");
$.append(odv);
// And many more

But I'm not sure if this is any better.

Source: (StackOverflow)

Is Safari on iOS 6 caching $.ajax results?

Since the upgrade to iOS 6, we are seeing Safari's web view take the liberty of caching $.ajax calls. This is in the context of a PhoneGap application so it is using the Safari WebView. Our $.ajax calls are POST methods and we have cache set to false {cache:false}, but still this is happening. We tried manually adding a TimeStamp to the headers but it did not help.

We did more research and found that Safari is only returning cached results for web services that have a function signature that is static and does not change from call to call. For instance, imagine a function called something like:

getNewRecordID(intRecordType)

This function receives the same input parameters over and over again, but the data it returns should be different every time.

Must be in Apple's haste to make iOS 6 zip along impressively they got too happy with the cache settings. Has anyone else seen this behavior on iOS 6? If so, what exactly is causing it?

The workaround that we found was to modify the function signature to be something like this:

getNewRecordID(intRecordType, strTimestamp)

and then always pass in a TimeStamp parameter as well, and just discard that value on the server side. This works around the issue. I hope this helps some other poor soul who spends 15 hours on this issue like I did!

Source: (StackOverflow)