In JavaScript, you can declare and define a function using the function keyword followed by the function name and parentheses. You can also specify parameters within the parentheses if the function accepts any. Here's an example:
// Function declaration
function greeting() {
console.log("Hello!");
}
// Function definition with parameters
function addNumbers(num1, num2) {
return num1 + num2;
}
In the above example, greeting is a function that doesn't accept any parameters and logs "Hello!" to the console. addNumbers is a function that accepts two parameters (num1 and num2) and returns their sum.
You can also assign a function to a variable, known as function expressions:
// Function expression
const greeting = function() {
console.log("Hello!");
};
// Function expression with arrow function syntax (ES6)
const addNumbers = (num1, num2) => {
return num1 + num2;
};
In this case, greeting and addNumbers are variables that store anonymous functions. The function expressions are assigned to these variables using either the function keyword or the arrow function syntax (ES6).