JS Basic knowledge

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• 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

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< 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