JS variable and its scope (with examples and EXPLANATIONS)

javascript, Unlike Java and c, is a non-type, weak-detection language. It does not need to declare variable types, and we can assign various types of data to the same variable as long as they are in the form of a Value.

Tools/materials

• Chrome browser (the following code tests are all debugged by Chrome)

Method/step

• JS variable type and declaration method, etc., This article no longer explain, the reader can refer to the relevant official documents

  This article mainly introduces the JS variable and its scope

• The scope of JS variables can be divided into: "global variables" and "local variables"

  "global variables": declaring variables outside of a function

  "local Variable": declares a variable in the body of the function and is accessible only within the body of the current function, such as: function () {var a = 0;}

  Note: variables that are declared are generally not the Var keyword, and the directly assigned variable is a global variable

• Here are some small examples to familiarize yourself With.

  1. Function Test () {

  A = 30;

  var B = 20;

  }

  Test ();

  Console.log ("a=" +a); It's obvious here that A is a global variable

  Console.log ("b=" +b);//b is a local variable, so in the function test out of the box, the hint is undefined

• 2.

  var a = 1;

  Function Test () {

  Console.log ("a=" +a); Here A is undefined

  The variables declared in the/* function are defined throughout the Function. If there is a defined variable inside the function, even if it is output before the definition, it executes the later definition statement and then determines the output, so that the declared variable will work in the whole Function. */

  var a = 2;

  }

  Test ();

• 3.

  For the comparison of two small examples:

  var a; function Fun () {a = "global";} console.log (a);//output undefined

• var a; function Fun () {a = "global";}

  Fun (); Console.log (a);//output Global

• For these two small examples, the only difference is that one executes the fun function and one is not executed;

  If it is

  var a;console.log (a);//because A is defined but not initialized, the output undefined

  and function Fun () {...} The initialization operation is in the scope of the fun function, and if you do not execute the fun ()

• 4, the function domain takes precedence over the global domain, therefore the local variable a will overwrite the global variable a

  var a=1;

  Function Main () {

  var a=2;//local variables

  Console.log (A);//2

  }

  Main ();

  Console.log (A);//1

• 5. JavaScript does not have a block-level scope

  Function Test () {

  for (var i = 0; i < 3; i++) {

  i=0,1,2, exit loop when last execution to i=3

  }

  Console.log (I);//3

  }

  Test ();

  Equivalent

  Function Test () {

  var i;

  For (i = 0; I < 3; i++) {

  i=0,1,2, exit loop when last execution to i=3

  }

  Console.log (I);//3

  }

  Test ();

JS variable and its scope (with examples and EXPLANATIONS)