What is a block?

A block is a statement (or compound statement) that combines zero or more statements. The block is defined by a pair of braces {}

Block declaration: {StatementList}

2. What is block-level scope?

When var or function declarations are made in non-strict mode, variables declared by var or functions created in non-strict mode have no block-level scope.

Here’s an example:

var x = 1; { var x = 2; } console.log(x); // The output is 2, because var has no block-level scopeCopy the code

while

let x = 1; { let x = 2; } console.log(x); // The output is 1. Let is block-scopedCopy the code

3. Comparison of let and const

Let and const both have block-level scope, as shown below:

If there were no block-level scope, the output would be 2, because there is block-level scope, so the output would be 1.

4. What is variable promotion?

Look at another blog post: Variable promotion

5. Var, let, const

Var, let, const comparison The same The difference between
var
let
const

  • Block-level scope

    • Var does not, let and const do.

  • Initialize the

    • Variables defined by const must be initialized and cannot be modified
    • The variable defined by var can be modified. If it is not initialized, it is undefined
    • Let is a block-level scope. If defined internally, let has no effect on the outside of the function.

  • Variable ascension

    • Let /const has no variable promotion
    • Var is variable enhanced

Personal understanding is very simple, welcome criticism, correction, thank you.