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Functions

A JavaScript function is a block of code designed to perform a particular task. A function is executed when "something" invokes it (calls it).

-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: {}

  • i.g
function name(parameter1, parameter2, parameter3) {
// code to be executed
}

1. Function **parameters** are listed inside the parentheses **()** in the function definition.
2. Function **arguments** are the **values** received by the function when it is invoked.
3. **NOTE**: 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)
    • i.g:
    document.addEventListener('click', e =>{
  //Some code
};
  • When it is invoked (called) from JavaScript code
    • i.g:
    let sum = getSum(12,24);
  • Automatically (self invoked)

-Function 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":

  • i.g:
//Calculate the product of two numbers, and return the result
var x = myFunction(4, 3);   // Function is called
return value will end up in x

function myFunction(a, b) {
  return a * b;             // Function returns the product of a and b
}

-Why Functions?

  1. Code Reusability:

We can reuse code: Define the code once, and use it many times. We can use the same code many times with different arguments, to produce different results.

  1. Code Organization:

We can organize our code and make it cleaner.

-The () Operator Invokes the Function

We can call a function by just typing the function name followed by () and the argument list if any. NOTE: Calling a function without () will return the function object instead of the function result.

-Functions Used as Variable Values

Functions can be used the same way as you use variables,in all types of formulas, assignments, and calculations.

  • i.g:
//Instead of using a variable to store the return value of a function
var x = toCelsius(77);
var text = "The temperature is " + x + " Celsius";
//You can use the function directly, as a variable value
var 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. -i.g:

// code here can NOT use carName

function myFunction() {
  var carName = "Volvo";
  // code here CAN use carName
}

// code here can NOT use carName

NOTE:

  • 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.