The target

• Be able to say why functions are needed (to make a lot of code reusable)
• Ability to write functions according to syntax
• Ability to encapsulate functions as required
• Be able to tell how parameters and arguments are passed
• The ability to use the return value of a function
• You can use arguments to get function arguments

You don’t need to read this article if you can answer the target’s questions

directory

• Concept of function
• Use of functions
• Parameters of a function
• The return value of the function
• The use of the arguments
• Function of case
• Two ways to declare a function

1. The concept of functions

A function is a block of code that encapsulates a call that can be repeated

Purpose: To make a lot of code reusable

2. Use of functions

Functions are used in two steps: declare the function and call the function.

• Function is the keyword for declaring functions and must be lowercase
• Since functions are defined to do something, we usually name them as verbs, such as getSum
• Don’t forget to add parentheses when calling
• Tip: if a function is not called, it is not executed

Function hanshuming () {// function body code}Copy the code

// Call the function name (); Execute the function body code by calling the function nameCopy the code

2.3 Function encapsulation

• Function encapsulation encapsulates one or more functions in the form of functions and provides only a simple function interface
• Simple understanding: Packaging is similar to integrating computer parts into a case (similar to express packaging)

3. Parameters of the function

3.1 Arguments and parameters of a function

When you declare a function, you add some parameters, called parameters, to the function name in parentheses. When you call the function, you also need to pass the corresponding parameters, called arguments.

What arguments do: Some values cannot be fixed inside a function. We can use arguments to pass in different values when calling a function.

Note:

• Parameters are separated by commas
• Parameters can be thought of as undeclared variables

3.3 Parameter Mismatch of function parameters

3.4 summary

• A function can take arguments or no arguments
• When declaring a function, the parameters inside the parentheses are parameters that default to undefined
• When you call a function, the arguments inside the parentheses are arguments
• Multiple parameters are separated by commas
• The number of parameters may not match the number of arguments, but the result is unpredictable, so we try to match

4. Return value of the function

4.1 return statement

A ternary expression that determines which of two numbers is the largest

Return num1 > num2? Num1: num2;

1. Termination of the function
2. Only one value can be returned, the last value
3. No value will return undefined

4.2 The maximum value of an array

function getArrMax(arr) { var max = arr[0]; for (var i=1; i<arr.length; i++){ if (arr[i] >max){ max = arr[i]; }} return Max} var re = getArrMax([5,2,99,101,67,77]); console.log(re);Copy the code

In our actual development, we often use a variable to accept the return result of a function to make it easier to use

4.5 Differences between break, Continue, and Return

• Break: to end the body of the current loop (for, while)
• Continue: To break out of the current loop and continue with the next loop (for, while)
• Return: Not only can you exit the loop, you can also return the value of the return statement, and you can also end the code inside the current function

Little homework

• Write a function where the user enters an arbitrary arithmetic operation of any two numbers (simple calculator small function) and can pop up the result of the operation.
• Write a function where the user enters the maximum value of any two numbers and returns the result of the pop-up operation.
• Write a function where the user enters the maximum value of any three different numbers and can single out the result of the operation.
• Write a function where the user inputs a number to determine if it is a prime and returns the value back.

5. Arguments

Arguments can be used to retrieve arguments when we are not sure how many arguments are passed. In JavaScript,arguments is actually a built-in object for the current function. All functions come with a built-in arguments object, which stores all arguments passed.

Arguments presentation form is a pseudo-array, so it can be iterated over. The pseudo-array has the following characteristics

• Has the length property
• Store data by index
• Methods push, pop, etc. without arrays

5.1 Use the function to find the maximum value of any number

function getMax() { var max = arguments[0]; for (var i=1 ; i< arguments.length ; i++) { if (max <arguments[i]) { max = arguments[i]; } } return max; } the console. The log (getMax (1, 2, 3));Copy the code

6. Function cases

6.1 Invert any array using function encapsulation

Function reverse(arr) {var newArr = []; for (var i = arr.length - 1; i >= 0 ; i--) { newArr[newArr.length] = arr[i]; Return newArr} var arr1 = reverse([1,3,4,6,9]); console.log(arr1);Copy the code

6.2 Bubble sort using function encapsulation.

Sort function sort(arr) {for (var I = 0; i < arr.length - 1; i++) { for (var j = 0; j < arr.length - i - 1; j++) if (arr[j] > arr[j + 1]) { var temp = arr[j]; arr[j] = arr[j + 1]; arr[j + 1] = temp; } } return arr; } var arr1 = sort([1, 10, 8, 3]); console.log(arr1);Copy the code

6.3 Functions You can call functions

Because each function is a separate block of code for a particular task, it is often used when functions call each other.

6.4 Determine how many days there are in February

Function backDay(){var year = prompt(' please enter the year '); if (isRunyear(year)){ alert('29'); } else { alert('28'); } } backDay(); Function isRunyear (year) {var flag = false; if (year % 4 == 0 && year %100 ! = 0 || year % 400 == 0){ flag = true; } return flag; }Copy the code

6.5 Two ways to declare functions

1. Custom functions using function keywords (named functions)

function fn (){}

Copy the code

2. Function expressions (anonymous functions)

Var var name = function() {}Copy the code

• Function expressions are declared in much the same way as variables, except that variables contain values and function expressions contain functions
• Function expressions can also pass parameters