JavaScript 中 var 和 let 的区别

var 和 let 都用于 javascript 中的变量声明，但它们之间的区别在于 var 是函数作用域，而 let 是块作用域。可以说，与 let 相比，用 var 声明的变量是在整个程序中定义的。一个例子将更好地阐明差异

变量示例：

Input:
console.log(x);
var x=5;
console.log(x);
Output:
undefined
5

让的例子：

Input:
console.log(x);
let x=5;
console.log(x);
Output:
Error

让我们看看 JavaScript 中的代码：代码 #1：

HTML

 

    
        // calling x after definition
        var x = 5;
        document.write(x, "\n");
 
        // calling y after definition
        let y = 10;
        document.write(y, "\n");
 
        // calling var z before definition will return undefined
        document.write(z, "\n");
        var z = 2;
 
        // calling let a before definition will give error
        document.write(a);
        let a = 3;
    

 

输出

代码#2：在下面的代码中，点击开始会调用一个函数，每0.5秒改变两个标题的颜色。第一个标题的颜色存储在 var 中，第二个使用 let 声明。然后在函数块外部访问它们。 Var 可以工作，但使用 let 声明的变量会显示错误，因为 let 是块作用域。

HTML

 

    Var vs Let

 

 
    GeeksForGeeks
    GeeksForGeeks
    Start
    
 
    
        function colour() {
 
            setInterval(function() {
 
                if (document.getElementById('var').style.color == 'black')
                    var col1 = 'blue';
                else
                    col1 = 'black';
 
                // setting value of color 1 through var
 
                if (document.getElementById('let').style.color == 'black') {
                    let col2 = 'red';
                } else {
                    col2 = 'black';
                }
 
                // setting value of color 2 through let
 
                document.getElementById('var').style.color = col1;
 
                document.getElementById('let').style.color = col2;
 
                // changing color of h1 in html
            }, 500);
 
        }
    

 

输出：

JavaScript 以网页开发而闻名，但它也用于各种非浏览器环境。您可以按照这个 JavaScript 教程和 JavaScript 示例从头开始学习 JavaScript。

让我们以表格形式了解差异 - ：

	var	let
1.	The var is a statement that is used to declare a variable	The let is also a statement that is used to declare a variable.
2.	Syntax -: var name = value;	The variables that are defined with let statement have a block scope.
3.	It takes parameters as a name of the variable and the value that we want to assign to our variable	We cannot declare a variable more than once if we defined that previously
4.	Example -: var websitename = “geeksforgeeks”;	Example -:  let x = 69;
5.	var is an ECMAScript1 feature.	let is a feature of ES6.
6.	Its supported browsers are -: Chrome , InternettheExplorer, Microsoft Edge, Firefox, safari , opera	Its supported browsers are -: Chrome49, Microsoft Edge12, firefox44 , safari11, opera36