JS declaration variable using var declaration vs. Var declaration not used

JS in the variable declaration of explicit and implicit declarations.

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VAR i=100;//Explicit declaration i=100;//Implicit declaration of

Variables that are explicitly declared using the var keyword in a function are local variables instead of the VAR keyword, which is declared as a global variable using the direct assignment method.
　
When we use access to a variable that is not declared, JS will make an error. And when we assign a value to a variable that is not declared, JS does not give an error, instead it thinks that we are implicitly declaring a global variable, and this must be noted

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var num = 1;

is to declare a variable in the current domain. If declared in a method, it is a local variable, or a global variable if declared in the global domain.

and

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num = 1;

is actually an assignment to a property. First, it tries to do so in the current scope chain (as declared in the method, the current scope chain represents global scope and method local scope etc ...). , if NUM is found in any current scope chain, the Num property is assigned, and if NUM is not found, it creates and assigns the NUM attribute in the global object (that is, the topmost object in the current scope chain, such as the Window object).

Attention! Instead of declaring a global variable, it creates a property of a global object.

Even so, it may be difficult for you to understand the difference between the variable declaration and the Create object attribute. In fact, JavaScript variable declarations, creation attributes, and each property in each JavaScript have flags that indicate their properties----such as read-only (ReadOnly) Dontenum (Dontdelete), and so on.

Because a variable declaration takes a property that is not able to be deleted, the comparison of var num = 1 with num = 1, which is a variable declaration, with a property that cannot be deleted, and therefore cannot be deleted, is a property of the global variable and can therefore be removed from the global variable.

You can declare JavaScript variables by using the VAR statement:

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var x; var carname; <!--[If!supportlinebreaknewline]--> <!--[endif]-->

After the above declaration, the variables do not have a value, but you can assign values to the variables when you declare them:

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var x=5; var carname= "Volvo";

Note: When assigning a text value to a variable, enclose the value in quotation marks.

Assigning values to JavaScript variables
To assign a value to a JavaScript variable by an assignment statement:

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x=5; Carname= "Volvo";

The variable name is to the left of the = symbol, and the value you want to assign to the variable is to the right of =.

After the above statement is executed, the value saved in the variable x is 5, and the value of Carname is Volvo.
Assigning values to an undeclared JavaScript variable

If the variable you are assigning has not been declared, the variable is declared automatically.
These statements:

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x=5; Carname= "Volvo";

The same effect as these statements:

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var x=5; var carname= "Volvo";

Cases

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NUM1 is a global variable and num2 is a property of window var num1 = 1; num2 = 2; Delete num1; Cannot delete Delete num2; Delete function model () { var num1 = 1; Local variables num2 = 2; Window's Properties anonymous functions (function () { var num = 1; Local variables NUM1 = 2; Inheriting scopes (closures) NUM3 = 3; Window's Properties }()) }