掌握 JavaScript 中的函數

Functions are fundamental building blocks in JavaScript that allow you to encapsulate code and reuse it throughout your program. In this blog, we'll cover the basics of functions, including function declarations, local and outer variables, parameters, default values, returning values, naming conventions, and the importance of comments. Let's dive in!

Function Declaration

A function declaration defines a named function with a specific set of parameters.

Syntax:

function functionName(parameters) {
  // code to execute
}

Example:

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

greet("Alice"); // Output: Hello, Alice!

Local Variables

Variables declared inside a function are local to that function and cannot be accessed outside of it.

Example:

function calculateSum(a, b) {
  let sum = a + b; // Local variable
  return sum;
}

console.log(calculateSum(3, 4)); // Output: 7
// console.log(sum); // Error: sum is not defined

Outer Variables

Functions can access variables declared outside of them, known as outer variables.

Example:

let message = "Hello, World!";

function printMessage() {
  console.log(message); // Accessing outer variable
}

printMessage(); // Output: Hello, World!

Parameters

Parameters are placeholders for the values that will be passed to the function when it is called.

Example:

function multiply(a, b) {
  return a * b;
}

console.log(multiply(2, 3)); // Output: 6

Default Values

You can provide default values for function parameters in case they are not passed when the function is called.

Example:

function greet(name = "Guest") {
  console.log("Hello, " + name + "!");
}

greet(); // Output: Hello, Guest!
greet("Bob"); // Output: Hello, Bob!

Returning a Value

Functions can return a value using the return statement.

Example:

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

let result = add(5, 3);
console.log(result); // Output: 8

Naming a Function

Function names should be descriptive and follow the camelCase convention.

Example:

function calculateArea(radius) {
  return Math.PI * radius * radius;
}

console.log(calculateArea(5)); // Output: 78.53981633974483

Functions == Comments

Comments are essential for documenting the purpose and usage of functions. They help other developers (and your future self) understand the code.

Example:

/**
 * Calculates the area of a circle.
 * @param {number} radius - The radius of the circle.
 * @return {number} The area of the circle.
 */
function calculateArea(radius) {
  return Math.PI * radius * radius;
}

console.log(calculateArea(5)); // Output: 78.53981633974483

Summary

• Function Declaration: Defines a named function with parameters.
• Local Variables: Variables declared inside a function are local to that function.
• Outer Variables: Functions can access variables declared outside of them.
• Parameters: Placeholders for values passed to the function.
• Default Values: Provide default values for parameters.
• Returning a Value: Functions can return a value using the return statement.
• Naming a Function: Use descriptive names and follow the camelCase convention.
• Functions == Comments: Document functions with comments to explain their purpose and usage.

Conclusion

Functions are a powerful feature in JavaScript that allow you to organize and reuse code effectively. By understanding how to declare functions, use local and outer variables, work with parameters and default values, return values, name functions appropriately, and document them with comments, you'll be able to write more modular and maintainable code. Keep practicing and exploring to deepen your understanding of functions in JavaScript.

Stay tuned for more in-depth blogs on JavaScript! Happy coding!

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