A fumction is someting more than data A function is a block of code designed to perform a particular task. A function is executed when «something» invokes it (calls it). function myFunction(p1, p2) { return p1 * p2; // The function returns the product of p1 and p2 } JavaScript Function Syntax A JavaScript function is defined with the function keyword, followed by a name, followed by parentheses (). Function names can contain letters, digits, underscores, and dollar signs (same rules as variables). The parentheses may include parameter names separated by commas: (parameter1, parameter2, …) The code to be executed, by the function, is placed inside curly brackets: {} function name(parameter1, parameter2, parameter3) { // code to be executed } Function parameters are listed inside the parentheses () in the function definition. Function arguments are the values received by the function when it is invoked. Inside the function, the arguments (the parameters) behave as local variables. Function Invocation The code inside the function will execute when «something» invokes (calls) the function: When an event occurs (when a user clicks a button) When it is invoked (called) from JavaScript code Automatically (self invoked) var x = myFunction(4, 3); x es consciente de su valor esto es una conciencia simple document.getElementById(«demo»).innerHTML = x; function myFunction(a, b) { return a * b; } The () Operator Invokes the Function Using the example above, toCelsius refers to the function object, and toCelsius() refers to the function result. Accessing a function without () will return the function definition instead of the function result: Example function toCelsius(fahrenheit) { return (5/9) * (fahrenheit-32); } document.getElementById(«demo»).innerHTML = toCelsius; Source: