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JQuery interview questions and answers

JavaScript is the standard language for client-side scripting, while jQuery makes writing JavaScript easier. You can do more with just a few lines of jQuery code. It is one of the longest-used JavaScript libraries, and there are few new projects that use native JavaScript instead of jQuery. This means that as a Java web developer, you will find a lot of jQuery interview questions in a Java web development interview.

Earlier, most of them were HTTP, HTML, CSS and JavaScript, but recently, in addition to the basics of JavaScript, people also want to know if you are familiar with jQuery. These 16 jQuery questions are for web developers and are also very convenient for you to correct some key concepts before attending a phone or video round of interview. If you are new to jQuery, it can also help you understand the basics better and inspire you to discover more.

1. What is the $() in the jQuery library? (the answer is as follows)

The $() function is another name for the jQuery () function, which is weird at first glance and makes the jQuery code obscure. Once you get used to it, you will love its simplicity. The $() function is used to wrap any object into a jQuery object, and you are then allowed to call several different methods defined on the jQuery object. You can even pass a selector string to the $() function, which returns a jQuery object containing an array of all matching DOM elements. I have seen this issue mentioned several times, although it is very basic, and it is often used to distinguish whether a developer knows jQuery or not.

two。 There are five on the web page.

Elements, how do I use jQuery to select them? (answer)

Another important jQuery problem is based on selectors. JQuery supports different types of selectors, such as ID selector, class selector, tag selector. Since ID and class are not mentioned in this problem, you can use the tag selector to select all p elements. The jQuery code: $("p"), which returns a jQuery object containing all five p tags. For more detailed answers, see the article linked above.

3. What is the difference between the ID selector and the class selector in jQuery? (answer)

If you've ever used CSS, you probably know the difference between ID selector and class selector, and so does jQuery. The ID selector uses ID to select elements, such as # element1, while the class selector uses CSS class to select elements. Use the ID selector when you only need to select one element, and use the class selector if you want to select a set of elements with the same CSS class. There is a good chance that you will be asked to use ID selector and class selector to write code during the interview process. The following jQuery code uses the ID selector and the class selector:

$('# LoginTextBox') / / Returns element wrapped as jQuery object with id='LoginTextBox'$ ('.active') / / Returns all elements with CSS class active.

As you can see, another difference between ID selectors and class selectors from a syntax perspective is that the former uses the character "#" while the latter uses the character "." For more detailed analysis and discussion, see the answer link above.

4. How to use jQuery to hide a picture when a button is clicked?

This is an event handling problem. JQuery provides good support for events such as button clicks. You can use the following code to hide an image located through ID or class. You need to know how to set the event for the button and execute the hide () method, as shown in the following code:

$('# ButtonToClick') .click (function () {$('# ImageToHide') .hide ();})

5. What function is $(document) .ready ()? Why use it? (answer)

This question is very important and is often asked. The ready () function is used to execute code when the document enters the ready state. JQuery allows you to execute code when the DOM is fully loaded (for example, when the HTML is fully parsed and the DOM tree is built). The biggest advantage of using $(document). Ready () is that it works for all browsers, and jQuery helps you solve cross-browser problems. Users who need to know more can click on the answer link to view the detailed discussion.

6. How is the JavaScript _ window.onload event different from the jQuery ready function? (answer)

This question and answer is followed by the last one. The main difference between the JavaScript _ window.onload event and the jQuery ready function is that in addition to waiting for DOM to be created, the former waits for all external resources, including large images, audio, and video, to be fully loaded. If it takes a lot of time to load images and media content, users will feel a significant delay in the execution of the code defined on the _ window.onload event.

On the other hand, the jQuery ready () function only waits for the DOM tree, not for images or external resources to load, which makes it faster to execute. Another advantage of using jQuery $(document). Ready () is that you can use it multiple times in a web page, and the browser will execute them in the order in which they appear on the HTML page, whereas for onload technology, it can only be used in a single function. Given this benefit, it is better to use the jQuery ready () function than the JavaScript _ window.onload event.

7. How do I find the selected items for all HTML select tags? (the answer is as follows)

This is one of the thorny jQuery questions in the interview. This is a basic question, but don't expect every jQuery beginner to know about it. You can get all the selections with multiple=true tags using the following jQuery selector:

$('[name=NameOfSelectedTag]: selected')

This code uses a combination of a property selector and a: selected selector, and the result returns only the selected options. You can modify it as needed, such as using the id attribute instead of the name attribute to get the tag.

8. What function is each () in jQuery? How do you use it? (the answer is as follows)

The each () function is like an Iterator in Java. It allows you to iterate through a collection of elements. You can pass a function to the each () method, and the called jQuery object executes the passed function on each of its elements. Sometimes this question is followed by the above question, for example, how to display all the selected items in the alert box. We can use the above selector code to find all the selected items, and then we print them one by one with the each () method in the alert box, as follows:

$('[name=NameOfSelectedTag]: selected') .each (function (selected) {alert ($(selected). Text ());})

Where the text () method returns the text of the option.

9. How do you add a HTML element to the DOM tree? (the answer is as follows)

You can use the jQuery method appendTo () to add a HTML element to the DOM tree. This is one of the many ways to manipulate DOM provided by jQuery. You can use the appendTo () method to add an existing element or a new HTML element to the end of the specified DOM element.

10. Can you use jQuery code to select all the hyperlinks inside the paragraph? (short answer)

You can use the following jQuery code snippet to select all nested paragraphs (

Tags) internal hyperlinks (tags)

$('p a')

11. What is the difference between the $(this) and this keywords in jQuery? (the answer is as follows)

This is a thorny problem for many jQuery beginners, but it is actually a simple one. $(this) returns a jQuery object on which you can call multiple jQuery methods, such as getting the text with text (), getting the value with val (), and so on. This represents the current element, which is one of the JavaScript keywords and represents the current DOM element in the context. You cannot call the jQuery method on it until it is wrapped in a $() function, such as $(this).

twelve。 How do you use jQuery to extract the attributes of a HTML tag, for example. Linked href? (answer)

The attr () method is used to extract the value of an attribute of any HTML element. You first need to use jQuery to select and select all links or a specific link, and then you can use the attr () method to get the value of their href attribute. The following code finds all the links on the page and returns the href value:

$('a') .each (function () {alert ($(this) .attr ('href'));})

13. How do you use jQuery to set a property value? (answer)

An additional follow-up to the previous problem is that the attr () method has more than the same capabilities as other methods in jQuery. If you call attr () with a value.

Object .attr ("name", "value"); name is the name of the property, and value is the new value of this property

Object .prop ("name", "value")

Set multiple attribute values: object .attr ("name": "value", "name": "value") attribute: attribute value, attribute: attribute value

The difference between attr and prop in jquery

For fixed attributes (attributes that are inherent in the html element), you can use the prop method to manipulate attributes of Boolean type when processing

For dom attributes defined by ourselves for html elements, you cannot use the attr method to manipulate Boolean attributes when dealing with them.

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The dom attribute values of the element in this example are "id, href, class and action". Obviously, the first three are inherent attributes, while the latter action attribute is defined by ourselves.

The element itself has no attributes. This is the custom dom property. When dealing with these properties, it is recommended that you use the attr method, which returns an undefined value when you use the prop method to take values for custom properties and to set property values.

For elements like checkbox,radio and select, select attributes that correspond to "checked" and "selected". These are also inherent attributes, so you need to use the prop method to get the correct answer.

14. What is the difference between the detach () and remove () methods in jQuery? (answer)

Although both the detach () and remove () methods are used to remove a DOM element, the main difference between the two is that detach () keeps track of the previously released element, so it can be undone, while the remove () method maintains a reference to the previously removed object. You can also take a look at the appendTo () method used to add elements to DOM.

15. How do you use jQuery to add and remove CSS classes from an element? (answer)

By using the two jQuery methods, addClass () and removeClass (). Dynamically changing the class attribute of an element can be as simple as. Use the class ".active" to mark their inactivation and activation status, and so on

.addClass ("class name") add element. remove () remove style class

16. What are the main advantages of using CDN to load jQuery libraries? (answer)

This is a slightly more advanced jQuery problem. Well, apart from the benefits of saving server bandwidth and faster downloads, the most important thing is that if the browser already downloads the same jQuery version of the class from the same CDN, it won't download it again. Therefore, nowadays, many public websites use jQuery for user interaction and animation, and if the browser already has a downloaded jQuery library, the site will have a very good opportunity to show it.

17. What is the difference between the jQuery.get () and jQuery.ajax () methods?

The ajax () method is more powerful and configurable, allowing you to specify how long to wait and how to handle errors. The get () method is a specialized method that only gets some data.

18. What is the method chain in jQuery? What are the benefits of using the method chain?

The method chain calls another method on the result returned by one method, which makes the code simple and straightforward, and the performance is better because there is only one round of lookups on the DOM.

19. What if you return false in a jQuery event handler?

This is usually used to prevent events from bubbling up.

20. Which is more efficient: document.getElementbyId ("myId") or $("# myId")?

The first, because it calls the JavaScript engine directly.

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