Function

Info:

• A JavaScript function is a block of code designed to perform a particular task.

• A JavaScript function is executed when "something" invokes it (calls it).

• The code to be executed, by the function, is placed inside curly brackets: {}

  function myFunction(p1, p2) {
    return p1 * p2;
  }

• Syntax

  • defined with the function keyword, followed by a name, followed by parentheses ()

  • The parentheses may include parameter names separated by commas: (parameter1, parameter2, ...)

  • 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 name(parameter1, parameter2, parameter3) {
      // code to be executed
    }

• Invocation, function will execute when...

  • an event occurs (buttonClick)
  • called from JavaScript code
  • automatically (self invoked)

• return

  • When JavaScript reaches a return statement, the function will stop executing.

  • If the function was invoked from a statement, JavaScript will "return" to execute the code after the invoking statement.

  • Functions often compute a return value. The return value is "returned" back to the "caller":

    // Function is called, the return value will end up in x
    let x = myFunction(4, 3);
    
    function myFunction(a, b) {
    // Function returns the product of a and b
      return a * b;
    }

• Used as Variable Values

  • Instead of using a variable to store the return value of a function:

    let x = toCelsius(77);
    let text = "The temperature is " + x + " Celsius";

    You can use the function directly, as a variable value:

    let text = "The temperature is " + toCelsius(77) + " Celsius";

• Local Variables

  Variables declared within a JavaScript function, become LOCAL to the function.

  Local variables can only be accessed from within the function.

  // code here can NOT use carName
  
  function myFunction() {
    let carName = "Volvo";
    // code here CAN use carName
  }
  
  // code here can NOT use carName

  Since local variables are only recognized inside their functions, variables with the same name can be used in different functions.

  Local variables are created when a function starts, and deleted when the function is completed.

Danger:

• Accessing a function with incorrect parameters can return an incorrect answer:

  function toCelsius(fahrenheit) {
    return (5/9) * (fahrenheit-32);
  }
  let value = toCelsius(); // missing parameter

• Accessing a function without () returns the function and not the function result:

  function toCelsius(fahrenheit) {
    return (5/9) * (fahrenheit-32);
  }
  let value = toCelsius;
  // returns: function toCelsius(f) { return (5/9) * (f-32); }