1. Block-level scope

ES5 only has global scope and function scope, but no block-level scope.

Variables declared outside the function are global variables, which can easily lead to variable conflicts. Therefore, we have to use execute function immediately to avoid conflicts with global variables. In addition, variables used for counting within the loop will also be leaked into global variables.

To this end, ES6 introduces the concept of block-level scope.

2. let declare variables

  1. 1 Variables declared with let are only valid within the code block in which they are located (block-level scope).
	{
	    var a = 1;
		let b = 2;
	}
	a //1 
	b // Uncaught ReferenceError: b is not defined
  1. 2 The counter of the for loop is declared using the let command.
	for (let i = 0; i < 10; i++) {
		//
	}
	console.log(i);  //ReferenceError: i is not defined
  1. 3 There is no variable promotion. Variables should follow the `declare first and then use’ principle.

Variables declared with var are hoisted to the top of the scope and can be used before the variable is declared, with the value undefined. The variable declared by let does not have variable promotion. If it is used before the declaration, an error will be reported.

  1. 4 Temporary dead zone (TDZ):

Once a variable is declared with let inside a block scope, that binding only exists in that scope and is not affected by outer scopes. The variable cannot be accessed before the let declaration within that scope, even if a variable with the same name exists outside it.

	let a = 1;
	if (true) {
		// TDZ starts
		console.log(a); //Uncaught ReferenceError: a is not defined

		let a;  // TDZ ends
	}

	let b = 1;
	if (true) {
		let b = 100; 
		console.log(b);  //100
	}

You can understand the temporary dead zone this way: as soon as you enter the current block-level scope, all the variables to be used already exist, but they are not yet available. You can only obtain and use the variable after it is declared.

  1. 5 Variables cannot be declared repeatedly

Within the same scope, the let command cannot declare the same variable repeatedly.

	function () {
		let x = 10;
		var x = 20;
	}
	// Uncaught SyntaxError: Identifier 'x' has already been declared

3. const declares constants

  1. 1 ES6 introduces the concept of constants. The const command is used to declare constants, usually using uppercase letters to represent constants. Once declared, the value of a constant cannot be changed.
	const PI = 3.1415;
	PI = 3; //Uncaught TypeError: Assignment to constant variable.
  1. 2 When declaring a variable, it must be assigned a value immediately. Otherwise, an error will be reported.
	const SIN //Uncaught SyntaxError: Missing initializer in const declaration
  1. 3 The scope of const is the same as the let command: a constant is only valid within the block-level scope in which it is declared.
	if (true) {
	  const A = 5;
	}
	A // Uncaught ReferenceError: A is not defined
  1. 4 The constants declared by the const command are not promoted. There is also a temporary dead zone and can only be used after the declared position. At the same time, repeated statements cannot be made.