The difference between variables defined by const, var and let in JavaScript

1. Const declares a constant (The value pointed to by const is unchanged, so it must be initialized, otherwise an error will be reported, while the values ​​pointed to by var and let are variable and can be reassigned. You can Not initialized)

## Correct way of writing: const h = 'nnnn';

Incorrect way of writing: const h;

2 , var and let declare variables. The difference between the two is the scope

(1), var

Example 1:

function fun(){
        	var a=5;
        	console.log(a);
 }

Note : If placed inside a function, it is a local variable. Variable a can only be used within the function fun(), otherwise an error will be reported

Example 2:

<p id=&#39;p&#39;>
		<li>hahah1</li>
		<li>hahah2</li>
		<li>hahah3</li>
		<li>hahah4</li>
</p>

var li =document.getElementById('p').getElementsByTagName('li');
        for(var i = 0; i < li.length; i++) {            
            li[i].onclick =function(event) {
                console.log(&#39;li : &#39;+i+&#39;  &#39;+this.innerHTML);
            }
        }

Note: Example 2, it is a global variable, so each click corresponds to the same i

(2), let declares a variable, statement or expression whose scope is limited to the block level

For example:

var li =document.getElementById('p').getElementsByTagName('li');
        for(let i = 0; i < li.length; i++) {            
            li[i].onclick =function(event) {
                console.log(&#39;li : &#39;+i+&#39;  &#39;+this.innerHTML);
            }
        }

Note: The i in this place can only be in the for loop statement block Use

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