The difference between "Var" and "Var" when declaring in javascript, = = and = = =

1. The difference between "Var" and "Var" when declaring in JavaScript

When JavaScript declares variables

var a = 111;

And

A = 111;

Is it the same in two different ways?

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var a = 11;function test4 () {var a = 22;} Test4 (); Console.log (a);

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What was the result? 11

This is a good understanding that the Var a declaration inside the function is an internal variable, and the result is the value of the first A.

Change as follows:

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var abc = 11;function test4 () {abc = 22;} Test4 (); Console.log (ABC);

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What was the result? 22

Further changes:

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function test4 () {var AAA = 22;} Test4 (); Console.log (AAA);

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What's the result? Run an Error! REFERENCEERROR:AAA is not defined!

Change:

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function test4 () {var AAA = 22;} Test4 (); Console.log (TEST4.AAA);

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Operation will not error, the output is undefined.

Variables declared within a function or object construct are private. cannot be accessed externally. Includes the object after the prototype inherits.

But if so:

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function test4 () {bbb = 33;} Test4 (); Console.log (BBB);

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The result is 33.

This is the difference between a declaration with Var and no Var:

It's horrible. If a big project changes the value of BBB here, and does not add Var happens throughout the project global variable has a same name BBB is changed, no var is not only function in this function or object. It's hard to find out the mistake.

So write code must be cautious. Declaring variables to add can not be afraid of trouble. The result is completely different.

Conclusion: (1). The variable defined by Var in the scope of the function is a local variable, which is defined as a global variable without var.

Using the VAR definition

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var a = ' Hello world '; function bb () {var a = ' Hello Bill ';   Console.log (a);    }BB ()//' Hello Bill ' console.log (a); ' Hello World '

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Do not use VAR definitions

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var e = ' Hello world '; function cc () {e = ' Hello Bill ';    Console.log (e); ' Hello Bill '}cc ()//' Hello Bill ' Console.log (e)//' Hello Bill '

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(2). Under global scope, variables defined with Var cannot be deleted, and variables with no Var defined can delete. It also means that the implied global variable is strictly not a real variable, but rather a property of the global object, because the property can be removed by the delete and the variable is not available.

(3). using var to define variables also promotes variable declarations, which are

Use Var to define:

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function hh () {console.log (a); var a = ' Hello World ';} HH ()//undefined

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Do not use VAR definitions:

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function hh () {console.log (a); A = ' Hello World ';} HH ()//' A is not defined '

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This is the declaration premise of variables defined with Var.

2. js in the! =, = =,! = = =, the usage and the difference.

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var num = 1; var str = ' 1 '; var test = 1; Test = = num//true same type same value test = = num//true same type same value test!== num//false test same as NUM type, its value is the same, non-operation is definitely false num = = STR//true converts STR to numbers to check for equality. Num! = str//false = = Non-op num = = = str//false type is different, directly returning false num!== str//true num differs from the STR type means that its non-unequal operation is naturally Tru E.

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(1). The role of JavaScript "= ="

= = and! = Compare if the type is different, the first test conversion type, then the value comparison, the final return value comparison results.

1. When the content on both sides of = = is a string, the contents of the string are compared for equality.

2. When the content on both sides of = = is a number, the size of the number is equal.

3. When the content on both sides of = = is an object or a function property of an object, the memory address is compared for equality.

(2). = = and = = = Difference

= = = and!== values are compared only under the same type.

= = For general comparison, = = = for strict comparison, = = In comparison can be converted data type, = = = Strict comparison, as long as the type does not match the return flase.

Summarize:

= = and = = = Difference: "= =" only requires equal value; "= = =" Requires both values and types to be equal

The difference between "Var" and "Var" when declaring in javascript, = = and = = =