/*********1***********/
Null
Null is no stranger to programmers, indicating undefined (declared) variables
Undefined
Undefined represents a variable (assignment) that has been defined but not yet initialized
var foo = 1;
function Bar() {
foo = Ten;
Return
function Foo() {}//declaration in advance, so foo =10 when Foo is a local variable of bar
}
Bar ();
Console.log (foo); //1//Global variables to call
< Span class= "Hljs-title" >/*********2***********/
< Span class= "Hljs-title" >var f = function g () {
< Span class= "Hljs-title" > return 23;
< Span class= "Hljs-title" > }; Why G is not defined because the equals sign is preceded by a declaration variable, followed by an assignment
typeof G (); //g is not defined
/*********3***********/
< Span class= "Hljs-title" > var a = [0,1],b =[1,2];
Console.log (a+b)//0,11,2 when an array is added, the array is implicitly called by the ToString method, the array is converted to a string, and the + is the connector
JS Basic knowledge