What is the usage of js function

js function usage includes: 1. Declare function; 2. Call function; 3. Function parameters; 4. Function return value; 5. Anonymous function; 6. Function as parameter; 7. Function scope; 8. Recursive functions.

A JavaScript function is a reusable block of code that performs a specific task or calculation. They can receive input parameters and return output results. Functions play an important role in JavaScript. They can improve the readability, maintainability and reusability of code. This article will introduce the usage of JavaScript functions in detail.

1. Declaring a function:

In JavaScript, you can declare a function by using the function keyword. Function declaration includes function name, parameter list and function body. For example:

function add(a, b) {
   return a + b;
}

The above code declares a function named add, which receives two parameters a and b and returns their sum.

2. Calling a function:

To call a function, just use the function name followed by a pair of brackets and provide the required parameters in the brackets. For example:

var result = add(2, 3);
console.log(result); // 输出 5

The above code calls the previously declared add function and passes 2 and 3 as parameters to it. After the function is executed, the returned result is assigned to the result variable and output through console.log.

3. Function parameters:

The function can receive any number of parameters. In a function declaration, the parameter list is separated by commas. For example:

function greet(name) {
   console.log("Hello, " + name + "!");
}
greet("Alice"); // 输出 Hello, Alice!
greet("Bob"); // 输出 Hello, Bob!

The above code declares a function named greet, which receives a parameter name and outputs a greeting through console.log. When calling a function, you can pass different values ​​for parameters.

4. Function return value:

The function can use the return statement to return a value. The return value can be any JavaScript data type. For example:

function multiply(a, b) {
   return a * b;
}
var result = multiply(2, 3);
console.log(result); // 输出 6

The above code declares a function named multiply, which receives two parameters a and b and returns their product. When the function is called, the return value is assigned to the result variable.

5. Anonymous functions:

JavaScript also supports anonymous functions, that is, functions without function names. Anonymous functions can be declared using function expressions. For example:

var add = function(a, b) {
   return a + b;
}
var result = add(2, 3);
console.log(result); // 输出 5

The above code declares an anonymous function and assigns it to the variable add. This anonymous function can then be called like a normal function.

6. Functions as parameters:

JavaScript functions can also be passed as parameters to other functions. This usage is called functional programming. For example:

function calculate(a, b, operation) {
   return operation(a, b);
}
function add(a, b) {
   return a + b;
}
function multiply(a, b) {
   return a * b;
}
var result1 = calculate(2, 3, add);
console.log(result1); // 输出 5
var result2 = calculate(2, 3, multiply);
console.log(result2); // 输出 6

The above code declares a function named calculate, which receives three parameters a, b and operation. Depending on the operation function passed in, the calculate function can perform different calculation operations.

7. Function scope:

In JavaScript, functions have their own scope. This means that variables declared inside a function are not accessible outside the function. For example:

function sayHello() {
   var message = "Hello!";
   console.log(message);
}
sayHello(); // 输出 Hello!
console.log(message); // 报错，message is not defined

The above code declares a function named sayHello, and declares a variable named message inside the function. Attempting to access this variable outside the function will result in an error.

8. Recursive functions:

JavaScript functions also support recursive calls, that is, the function can call itself. This usage is very useful when solving some recursive problems. For example:

function factorial(n) {
   if (n === 0) {
       return 1;
   } else {
       return n * factorial(n - 1);
   }
}
var result = factorial(5);
console.log(result); // 输出 120

The above code declares a function named factorial, which calculates the factorial of a number. Inside the function, the factorial calculation is implemented by calling itself recursively.

The above are some common uses of JavaScript functions. Functions are a very important concept in JavaScript. Mastering the usage of functions can help us write more efficient and elegant code.

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