What is the difference between es5 and es6 scopes

Difference: There are only two types of scope in es5: global scope and function scope, while there are three types of scope in es6: global scope, function scope and block-level scope, with a new one added Block-level scope. The role of block-level scope: It can solve the problem of outer variables being overwritten due to the promotion of inner scope variables, and prevent variables used for loop counting from leaking into global variables.

The operating environment of this tutorial: windows7 system, ECMAScript version 6, Dell G3 computer

The scope of es5 and es6 Difference:

• There are only two scopes in es5: global scope and function scope

• Scope in es6 There are three types: global scope, function scope and block-level scope

In Es5, there are only global scope and function scope

ES5 Use var to declare variables. Variables declared with var may exist in the global scope or in the local scope. The specific situation is as follows

1. Global scope

Three situations of having global scope

a. Variables declared outside the function have global scope
b. Undefined variables with direct assignment automatically Declared as a global variable
c. The properties of the window object have global scope

2. Local scope (function scope)

The scope of variables in the function body

• Variables defined within the function can only be accessed within the function
  

Example

  var a = 1;
  console.log(a);// 1                  此处a为全局变量，在全局作用域下都可访问得到

  b = 2
  console.log(b); // 2                 此处b未被var定义，而是被直接赋值，自动声明为全局变量
  
  function fun() {
    var c = 3;
    console.log(c);//3                 此处c存在在函数作用域中，仅在函数fun中可访问
  }
  fun()
  console.log(c);// undefined         全局作用域下访问函数作用域中的变量c，得到undefined

New block-level scope in Es6

Block-level scope can be simply understood as: the content enclosed in curly brackets {}, it can Contains a scope of its own. Variables in block-level scope are declared by let and const

Why is block-level scope needed?

1. Solve the problem of outer variables being overwritten due to promotion of inner scope variables

var i = 5;
function fun(){
  console.log(i);//undefined
  if(true){
    var i = 6
    console.log(i);//6
  }
}
fun()

Execution results

The variable i in function fun is declared using var. This involves the issue of variable promotion. The so-called variable promotion means that function declarations and variable declarations are always quietly "promoted" to the top of the method body by the interpreter. So the i here is equivalent to reaching the top of function fun in advance, but the assignment is still performed when i = 6 is running. The above code is actually equivalent to:

var i = 5;
function fun(){
  var i;
  console.log(i);
  if(true){
    i = 6
    console.log(i)
  }
}
fun()

When the first i is printed , i is only declared but not assigned (i is assigned a value of 6 in the if statement), so the first printed i is undefined, and the second printed i is 6

var i = 5;
function fun(){
  console.log(i);//5
  if(true){
    let i = 6
    console.log(i);//6
  }
}
fun()

If used let declares the variable i in if, then the curly braces { } where the if statement is located will form a block-level scope, and the variables declared in this scope will be "bound" in this area and will no longer be affected by the outside. (i.e. temporary dead zone), so the first i output when executing the fun function is var i=5 in the global scope, and the i output in the if statement is let i=6## declared in the block-level scope.

#2. Prevent variables used for loop counting from leaking into global variables

for(var i = 0; i < 3; i++){
  doSomething()
}
console.log(i)//3

The above code declares the i variable with var for loops. Ideally, i should only be used in loops. It is valid in the body, but i here is exposed in the global scope, so after the loop ends, the value of i can still be accessed in the global scope

for(let i = 0; i < 3; i++){
  console.log(i)
}
console.log(i)//undefined

If you use block-level scope let to declare i, then the i variable declared here is only valid within the for loop curly braces { }. Accessing variables in the block-level scope in the global scope will result in undefined

Block-level scope features

1. Variables declared by let are only valid in the scope (within the current curly braces), so arbitrary nesting is allowed, at each level They are all separate scopes

2. The inner scope can have the same name as the outer scope variable (no scopes are used without interfering with each other)

3. let can only exist in the current scope Top level

Note: If there are variables/constants declared by let or const in { } in if statements and for statements, the scope of the { } also belongs to the block scope

Examples about scope

<script type="text/javascript">
	{
		var a = 1;
		console.log(a); // 1
	}
	console.log(a); // 1
	// 可见，通过var定义的变量可以跨块作用域访问到。

	(function A() {
		var b = 2;
		console.log(b); // 2
	})();
	// console.log(b); // 报错，
	// 可见，通过var定义的变量不能跨函数作用域访问到

	if(true) {
		var c = 3;
	}
	console.log(c); // 3
	for(var i = 0; i < 4; i++) {
		var d = 5;
	};
	console.log(i);	// 4   (循环结束i已经是4，所以此处i为4)
	console.log(d); // 5
	// if语句和for语句中用var定义的变量可以在外面访问到，
	// 可见，if语句和for语句属于块作用域，不属于函数作用域。

	{
		var a = 1;
		let b = 2;
		const c = 3;	
		
		{
			console.log(a);		// 1	子作用域可以访问到父作用域的变量
			console.log(b);		// 2	子作用域可以访问到父作用域的变量
			console.log(c);		// 3	子作用域可以访问到父作用域的变量

			var aa = 11;
			let bb = 22;
			const cc = 33;
		}
		
		console.log(aa);	// 11	// 可以跨块访问到子 块作用域 的变量
		// console.log(bb);	// 报错	bb is not defined
		// console.log(cc);	// 报错	cc is not defined
	}
</script>

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