JavaScript Introduction to Functions

Charles Russell

Bennu Bird Media

What is a function.

● As Defined by W3Schools A function is a block of code that will be executed when "someone" calls it.● Saved for later and use on demand

Function invocations

● functionName( parameterList);● Parameters, or arguments, are the values to be

used by the function– Parameters may be expressions, variables,

literals, or objects● Ex: alert('Hello World');● Functions may return values

– name = fullName('Charles', 'Russell');

Making Your Own Functions

● Functions can be constructed two ways● Statement form● Expression form

Statement form

function nameOfFunction(parameter1, parameter2, ...) { //statments and expressions

return result; //Optional if no value to be returned

}

● The return statement is optional and may or may not return a value

● Immediate invocation optional with () following end of code block

● If a parameter is declared and not passed no error is generated until an attempt is made to use the missing parameter.

● Some functions are selfcontained and need no arguments

Functions as Values

● Remember in the discussion on data types● Anything not a primitive is an object

● In JavaScript functions can be used as values, be passed as parameters, and returned as values from functions

Expression form

var nameOfFunction = function (parameter1, parameter2, ...) {

//statments and expressions

return result; //optional if no value to be returned

}

● Later we will see that in general this is more useful than the statement form.

Scope

● In this Language variables default to global● This means that any function anywhere can use and

overwrite your values at any time● The var keyword is what is used to limit scope to

the function in which the variable was delcare● There is no block scope only function scope

● This means that any local variable declared in a function can be accessed from anywhere in the function even sub functions

● Variables in parameters have local scope

this Keyword

● The keyword this is a special object that refers to different important objects based upon context.● When a function is created with the statement form

and is not part of some other Object this refers to the global Object if ES3 this is undefined if ES5 – For browsers this is the Window Object

● When function used as a constructor this refers to current instance of current function

● When as a method this refers to calling object (normally the parent)

Summary

● Function are code ment to be used later● A function is called by using its name and opening

and closing parents surround 0 or more parameters

● Functions may or may not return a value● Functions are objects and can be used as values

(not just the result)● If a declared parameter is missing or out of order

no error is generated● Functions may be declared in statement or

expression form● Variables default to global scope● var keyword used to limit variable to function

scope● This keyword may refer to the global object, the

calling object, the current instance of the running function or be undefined based upon context

Introduction to Objects