1. Variables

Variables store and retrieve values.

• Can take any type of data; string, array, integer, float, etc

2. Functions

A function is a list of instructions to accomplish a well-defined task.

• The JavaScript interpreter processes these instructions one by one
• A functions inputs is called it parameters e.g. someNumber
• A functions output is called the return value

• The above function definition accomplishes nothing by itself
• In order for it to be useful it must be put to work, like so:

3. Parameters & Arguments

As we have already learned, functions can have parameters.

• Function parameters are the names listed in the function definition
• Function arguments are the actual values passed to (and received by) the function

4. Objects

An object is a collection of variables and functions.

• Ideally, the components of an object are put together in such a way that the object represents both the attributes and behaviour of some “thing” being modelled in the program

An object used to represent a stopwatch might contain:

A variable to represent hours
A variable to represent minutes
A variable to represent seconds

Functions to manipulate those values:
- start()
- stop()

• Object variables are called “properties”
• Object functions are called “methods”

5. Properties

The values that make up an object are called the objects properties.

• Properties are variables that belong to an object, sometimes called “member variables”
• Other than the fact that properties belong to objects, there is little behavioural difference between variables and properties
• A property’s name alone is insufficient to identify an objects property
• This is because properties belong to objects, therefore the object to which the property belongs must also be specified:

stopwatch.hours

stopwatch.minutes

stopwatch.seconds

6. Methods

Methods are functions that belong to objects, sometimes called “member functions”.

• Member functions are designed to operate directly on the properties of the object to which they belong
• Here is a function which adds five minutes on to the current recorded time

• “this” is recognised as a synonym for “the object to which I belong”
• Since, at this point, the addOnFiveMinutes() function doesn’t actually belong to an object, the “this” doesn’t have much meaning
• We can use:

• This assignment tells the JavaScript interpreter to let update be a member function (or a method) of the stopwatch object
• The behaviour of stopwatch.update() should be the same as specified for addOnFiveMinutes()
• Now that stopwatch has an update() method, we can invoke it with the following statement:

stopwatch.update();

• This statement tells the JavaScript interpreter to run the addOnFiveMinutes() function with the word “this” replaced by “stopwatch”
• As a result, the stopwatch object’s member variables (or properties); “hours” and “minutes”, will be replaced with values that represent a time that is five minutes later