Defining Javascript Functions

Overview

• Prerequisite Topics
• What is a Function?
• What is an Input?
• What is an output_value?
• return statement
• contrasting return and print
• Using a Function

What is a Function?

• A function is a block of reusable instruction(s).
• A function relates an input to an output_value.
• The intent of a function is to re-execute the same lines of in different areas of a program.
• Functions provide better modularity for an application

Function vernacular

• The function signature is the first line of a function definition.
• A function signature is denoted by the keyword function, followed by the name of the function, followed by parenthesis (), followed by a {.
• A code block begins with an open curly bracket, {, and encapsulates several lines of code, followed by a closing curly bracket, }
• A function definition is a function signature and its respective code block.
• A function call is the execution of a function.

// this is outside the scope of the of the function code-block
function myFunctionName() { // this is the function signature
    output_value = "Hello world"; // this is the beginning of the function code-block
    print(output_value);// this is the ending of the function code-block
}  // this is outside the scope of the of the function code-block

Function Parameters

• Input parameters are denoted by the parenthesis ()
• In simple examples, an input is typically thought to be a numeric datatype or textual datatype
  • though, any type is valid as an input.

// this is outside the scope of the of the function code-block
function myFunctionName(input_value) { // this is the function signature
    output_value = "Hello " + input_value; // this is the beginning of the function code-block
    print(output_value); // this is the ending of the function code-block
} // this is outside the scope of the of the function code-block

return

• The return statement exits a function, optionally passing back an expression to the caller.
  • The expression passed back to the caller is named
• Omitting a return statement is the same as explicitly returning None

// this is outside the scope of the of the function code-block
function myFunctionName(input_value){ // this is the function signature
    output_value = "Hello " + input_value; // this is the beginning of the function code-block
    return output_value; // this is the ending of the function code-block
} // this is outside the scope of the of the function code-block

return vs print

• In contrast to print statements, return statements allow a system’s state change to be observable in a programmatic way
• In contrast to return statements, print statements allow a system’s state change to be observable in a human-readable

sing a function

• To make use of a function, you must make a call to the function.
• Calling a function only requires cognizance of the function signature: the name and parameters of the function.
  • You do not need to know how a function is defined to make a call to a function
• Below is an example of a function definition followed by a call to the function.

function greet(username) {
  let output = "Hello " + username
  console.log(output);
}


greet("Leon");
greet("Hunter");

• Script Output

Hello Leon
Hello Hunter