Functions can 'wrap' code with the same or similar logic, and execute the wrapped code logic through function calls. The advantage of this is that it streamlines the code and facilitates reuse.

The alert() and prompt() we used before are all JS functions, but they have been encapsulated and we can use them directly.

Basically the same as variable naming

Minimize camelCase nomenclature

The prefix should be a verb

Naming suggestions: Common verb conventions

Function name()

Note: The declared (defined) function must be called before it is actually executed. Use () to call the function

The function body is a component of the function. It is responsible for 'wrapping' corresponding or similar code. The code inside the function body will not be executed until the function is called. The function code of the function must be written in the function body

Formal parameters: When declaring a function name, what is written in parentheses to the right of the function name is called a formal parameter (formal parameter).

Actual parameters: When calling a function, the ones written in parentheses to the right of the function name are called actual parameters (actual parameters)

The formal parameters can be understood as variables declared within this function (for example, num1 = 10). The actual parameters can be understood as assigning values ​​to this variable.

When developing a summary, try to keep the formal parameters and actual parameter quantities consistent.

The alert ("print") and parselnt ("11") used are essentially parameters passed in function calls.

Formal parameter: can be regarded as a variable. A variable does not give a value. The default is: undefined

If the user does not enter actual parameters, NaN will appear

It can be improved. If the user does not enter actual parameters, the formal parameters can be given default values. The default value can be 0, so that the program is more rigorous.

The default value will only be executed when no actual parameters are passed, so if there are parameters, the passed actual parameters will be executed first.

Using the return keyword in the function body can transfer the internal execution results to the outside of the function for use

The code after return will not be executed and will end the current function immediately, so the data after return cannot be wrapped and co-written.

The return function can have no return. In this case, the function returns the default value undefined.

After the function is executed, the result is obtained, and the result is what the caller wants to get (the function does not need to output the result internally, but returns the result)

The execution results are more scalable and can be used by other programs.

The latter of two identical functions will overwrite the previous one.

In JS, the actual parameters of the formal parameters can be different.

Once the function encounters return, it will not continue to execute. Use return to end the function.

Global scope: Acts on the environment where all code is executed (inside the entire script tag) or an independent js file

Local scope: The code environment within the function is the local scope. Because it is related to functions, it is also called function scope.

Global variables: Global variables can be accessed and modified in any area

Local variables: Local variables can only be accessed and modified within the current function

As long as it is code, there must be at least one scope

Local scope written inside a function

If there is a function within a function, then another scope can be born in this scope.

Access principle: If you can access the local area first, if the local area is not available, then look for the global area.

function expression

Execute function immediately

Short circuit: only exists in && and ||. When certain conditions are met, the code on the right will not be executed.

Reason: The result of the entire equation can be obtained through the left side, so there is no need to judge the right side

Operation result: Regardless of && or ||, the operation result is the last executed expression, which is generally used in variable assignment.

Syntax: Boolean (content)

Memory: ‘’, 0, undefined, null, false, NaN are all false after being converted to Boolean values, and the rest are true

There is string addition ‘’ 1’, the result is ‘1’

Subtraction - (like most operations) can only be used in mathematics, it will convert the empty string '' to 0

null will become 0 after digital conversion

undefined will become NaN after digital conversion.