JavaScript functions are a fundamental building block of the language. They are used to define a block of code that can be executed multiple times with different inputs.

A JavaScript function can be defined using the "function" keyword followed by the function name, a list of parameters, and the code to be executed enclosed in curly braces.

For example, the following code defines a simple function called "add" that takes in two parameters, "a" and "b", and returns their sum:

function add(a, b) {

return a + b;

}

Functions can be invoked or called by using the function name followed by a set of parentheses, with the desired input values passed as arguments.

let result = add(2, 3);

console.log(result); // Output: 5

JavaScript also supports the concept of anonymous functions which are function without a name and are typically used as callbacks or immediately-invoked function expressions(IIFEs)

let add = function(a, b) {

return a + b;

};

console.log(add(2,3))// Output: 5

JavaScript also support Arrow function from ES6 which is another shorthand for anonymous function.

let add = (a, b) => {

return a + b;

};

console.log(add(2,3))// Output: 5

Functions in JavaScript are first-class citizens, which means that they can be treated like any other data type, such as numbers or strings, and can be assigned to variables, passed as arguments to other functions, or returned as values from functions.

JavaScript's support for higher-order functions, which are functions that take other functions as arguments or return them as values, makes it an extremely powerful and expressive language for functional programming.

In summary, Functions are an integral part of JavaScript, they can be defined and invoked using the "function" keyword, They can be anonymous and can be passed as arguments or returned as values.