javaScript data type

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

</body>
</html>
<script>
    //Weak type language
   /*Number 1, 2, -1, -2 console.log ("1" == 1) String 'string' "string" Boolean "True False Array array [1, 2, 3," BJT ", [" A "," B "]]] Brand is the array reference data type Object object {} 花括 {Name:" Millet ", AGE: 25} Reference Data Type undefined Not assigned NULL Empty // Declared Varuar Var A = 1; var b = "20"; var C = []; var D = {}; var A = 1, b = "23" var A, b = 2; b is 2, a is not 2; var A , B, c = 2 console.log (a); result is undefined console.log (b); result is undefined console.log (c); result is 2*/
    /*Variable name rules: 1. Compose from uppercase letters, numbers, underscores, dollar symbols ($) JQ class libraries 2. Can't start with numbers 3. Strict case sensation; IT and IT are two variables 4. Can't use The keywords in JS (there are special meaning in JS) and reserved words (JS is temporarily used, but there may be words that may be used in the future); 5. Try to see its name, know 6. A variable name consisting of multiple words, requires the use of a small hump nomenclature*/
    //Not how to use VAR now, use let or const
    //Scope: Global: Outside the function; local: In the function, VAR will get A this variable in the global, but not assigned
    //Const defined variables cannot be modified, and constants are given later.
    const arr=[]
    arr[0]='a'
    arr.pop()
    console.log(arr)
    //Do not affect the modification of the follow-up array
</script>
/* Variable Name Rules: 1. Composition of uppercase letters, numbers, underscores, dollar symbols ($) JQ class library 2. Unable to start 3. Strictly case sensitive; IT and IT are two variables 4. You can't use the keywords in JS (there is a special meaning in JS) and the reserved word (JS is temporary, but the words that may be used in the future) are named; 5. To see its name, know It is also a variable name consisting of multiple words, requires the use of a small hump nomenclature * / / now not using VAR, uses let or const // scope: Global: Outside the function; part: in the function Inside, VAR will get a variable in the global, but the variables defined by the value // Const cannot be modified, and the constant is given later, but when constant is an array, it can be modified.