jquery definition variable usage

In front-end development, it is often necessary to use the js library jquery to implement various functions. The definition and use of variables in jquery are also part of the knowledge we need to master. This article will explain the definition and use of variables in jquery.

1. jquery variable definition

In jquery, use the var keyword to define variables, as shown below:

// 定义一个变量
var num = 1; 

It also supports defining multiple variables at one time, using commas Just separate them:

// 定义多个变量
var num = 1, str = "hello world", bool = true;

It is worth noting that when defining variables in jquery, we do not need to initialize the variables, that is to say, the value of the variable can be assigned in subsequent code. As shown below:

// 定义一个变量，未初始化
var num;
num = 1; // 给变量赋值

2. The scope of jquery variables

In jquery, the scope of a variable refers to the accessible range of the variable. Variables defined inside a function can only be accessed within the function, while variables defined outside the function can be accessed by code within the entire file.

For example, in the following code, the variables num1 and num2 are defined inside the function, while the variable num3 is defined outside the function:

function myFunction(){
  var num1 = 1;
  var num2 = 2;
  console.log(num1); // 1
  console.log(num2); // 2
}

var num3 = 3;
console.log(num3); // 3
console.log(num1); // Uncaught ReferenceError: num1 is not defined
console.log(num2); // Uncaught ReferenceError: num2 is not defined

As can be seen from the above code, outside the function Accessing the variables num1 and num2 will result in an error because they are defined inside the function and are not accessible outside the function.

3. The use of jquery variables

In jquery, the use of variables is the same as in other languages, just use the variable name directly.

var num = 1;
console.log(num); // 1

At the same time, jquery also provides some special variables, such as $(this), $(document) and $(window).

$(this), represents the currently selected element, generally used in event functions:

$("button").click(function(){
  $(this).hide();
});

$(document), represents the entire HTML document, and can execute the code after the document is loaded. :

$(document).ready(function(){
  // 要执行的代码
});

$(window), represents the browser window, commonly used when scrolling the page:

$(window).scroll(function(){
  // 要执行的代码
});

4. Summary

This article introduces the definition and use of variables in jquery Methods, including variable definitions, scopes, and use of special variables. Mastering this knowledge can better use jquery to develop front-end applications and improve development efficiency.

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