JavaScript Functions

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

Function Syntax

A function is defined with the function keyword, followed by a name, followed by parentheses ().

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

• Function Name: Can contain letters, digits, underscores, and dollar signs (same rules as variables).
• Parameters: Names listed in the function definition.
• Arguments: The real values received by the function when it is invoked.

Function Invocation

The code inside a function is not executed until it is invoked. A function can be invoked when:

• An event occurs (like a user clicking a button).
• It is called from other JavaScript code.
• It is self-invoked (automatically).

The return Statement

When JavaScript reaches a return statement, the function will stop executing. If the function was called from a statement, JavaScript will "return" to execute the code after the invoking statement.

let x = myFunction(4, 3); // Function is called, return value will be stored in x

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

The () Operator

The () operator invokes the function. Accessing a function without () will return the function definition instead of the function result.

function toCelsius(f) {
  return (5/9) * (f-32);
}

let value = toCelsius(77); // Invokes the function
let definition = toCelsius; // Returns the function object

Key Points to Remember

• A function is a reusable block of code
• Parameters are placeholders; Arguments are real values
• The return statement sends a value back to the caller
• Functions help avoid code repetition (DRY - Don't Repeat Yourself)
• Local variables declared inside a function are only accessible within that function
• Invoking a function requires the () operator