The difference between VaR and let_ The difference between let and VaR

js Define variable foreword

1.js Defining variables Use var perhaps let

2.js Define constants Use const

3.var Variables are defined using js Language is born ; let Define variables from ES6 New syntax started .

4.let Defining variables , It conforms to the logic of variables defined by conventional strongly typed languages , The new definition can be used again + Can not repeat life .

var and let The difference between _let and var The differences are sorted out as follows ：

One 、 Different scope levels

var yes Function level scope

let Can only be Block level scope , Follow C# The scope of variables of equal strong type is the same .

Declared in the function var, The whole function is valid , For example, in for A defined in a loop var Variable , In fact, it is for It's also accessible outside the loop .

and let Because it's a block scope , So if the variable defined in the block scope , For example, in for Within the loop , It is not accessible outside , therefore for Circulation is recommended let.

code snippet ：

//for Cyclic {}  It's a block level scope 
for (var i = 0; i < 10; i++) { }
console.info(i); // Print ：10

for (let j = 0; j < 10; j++) { }
console.info(j); //j is not defin