Detailed explanation of let command in ES6

let is a new copy command in ES6. The let assignment command can only be called in the {} code block. The following is an example of the let command in es6. The specific content is as follows:

1. The usage of the let command is similar to the var command, but the variables declared by the let command are only valid within the code block where the let is located.

{
let a=10;
var b=1;
}
console.log(a);//Uncaught ReferenceError: a is not defined
console.log(b);

2. The let command does not have the phenomenon of "declaration in advance", so the variables must be declared first and then used

console.log(foo);
console.log(bar); 
var foo=2;//undefined
let bar=3;//Uncaught ReferenceError: bar is not defined

3. As long as the let command exists in the current block-level scope, the variables it declares The variable is bound to this block-level scope and is no longer affected by external influences

var tmp=123;
if(true){
tmp='abc';//Uncaught ReferenceError: tmp is not defined，因为块级作用域内存在let命令声明的相同变量，违反了let命令先声明后使用原则
let tmp;
}

4. The let command does not allow repeated declaration of the same variable in the same scope

function foo(){
let a=10;
let a=1;//Uncaught SyntaxError: Identifier 'a' has already been declared
}

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