Kulraj Suri

Software Engineer / Quant Dev

JavaScript Basic Definitions

1. Variables

Screen Shot 2016-08-02 at 18.09.47Variables 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

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

Screen Shot 2016-08-02 at 18.17.30

3. Parameters & Arguments

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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”




stopwatch stopwatch.hours (e.g. = 0) stopwatch.start()
stopwatch.minutes (e.g. = 24) stopwatch.stop()
 stopwatch.seconds (e.g. = 53)

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:



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

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  • “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:

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  • 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:


  • 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



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