This is the 24th day of my participation in Gwen Challenge

Global scope

In programming languages, whether Java or JavaScript, variables are generally divided into global variables and local variables. In JavaScript, a global variable is a variable that is mounted under the Window object, so you can use and access the global variable anywhere on the page. Let’s look at some code to illustrate what a global scope is.

var global = "global"
function getVar(){
	console.log(global)
	var local = "local"
	console.log(local)
}
getVar();
console.log(local) // 
console.log(global) //global
function setVar(){
	sVar = 'setVar'
}
setVar();
console.log(sVar)//setVar
console.log(window.sVar)//setVar
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You can see that the global variable can be accessed anywhere. It is the global variable. The local variable declared in getVar can only be accessed in getVar, and is null outside the function. In the following setVar function, although sVar is in the function, but there is no keyword declared by var, the directly assigned value is also a global variable, which can also be accessed externally.

Global variables are actually mounted on the window. Global variables are also accessible. There is also a problem with global variables, which can cause variable name conflicts.

Function scope

In JavaScript, a variable defined in a function is called a function variable, and it can only be accessed inside the function, so its scope is called the function scope.

function getVar(){
	var local = "local"
	console.log(local)
}
getVar();//local
console.log(local) // 
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As you can see, local is defined in the function getVar, so local is a local variable whose scope is in getVar, also called function scope.

This means that you can’t access the local variable except inside the function itself. When the function is finished, the local variable will be destroyed by the response, so it’s useless to access the local variable outside of the getVar function.

Block-level scope

ES6 has a new block-level scope, most directly reflected by the let keyword. Variables defined by the let keyword can only be accessed in the block-level scope, and cannot be used before they are defined.

For example, after the if statement and for statement {… } contains the block-level scope.

console.log(a)
if(true){
	let a = 'a'
	console.log(a)//a
}
console.log(a)//a is not defined
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You can see that it is ok to print a inside the if block, but outside the if block it says A is not defined.