JavaScript Notes:

I have kept these notes as I go through JavaScript and decided to post them because it may answer some questions.

• There is a client-side JavaScript and a server side JavaScript - we are looking only at the client side
• JavaScript can be embedded in either the header or the body. The browser uses its JavaScript interpreter when it encounters JavaScript. One concept of when to put things in the head and when to put things in the body is based on purpose. Code that reacts to users or produces on demand such as functions are usually in the head. Things that are interpreted as they are encountered and only once are in the body. They are handled as the drop-though of the HTML document progresses. Things in the head are not interpreted when they are encountered - they are acted on when something in the body demands their execution.
• JavaScript does not have classes and therefore cannot be an object-oriented language. It does have objects and the objects can have properties and methods.
• It is best to use <script language = "JavaScript"> rather than rely on the fact that JavaScript is the default script.
• JavaScript has five primitive types: Number, String, Boolean, Undefined and Null. There are predefined objects of types Number, String and Boolean which are called wrapper types and provide properties and methods for the objects.
• JavaScript is dynamically typed meaning that a variable type is determined by the interpreter. You can declare a variable either by using var to declare it or by simply assigning it a value.
• JavaScript supports unary operators (although I don't use them)
• JavaScript does implicit type conversions attempting to covert to the required type..
• You can use the length property with the string object to determine the length of a string. There are other string methods including toLowerCase, toUpperCase, substring, charAt, indexOf.

var thename="Smith/John";

var thelen = thename.length;

thename.charAt(5) will return the / (note that countin starts at 0)

thename.indexOf('/') will return 5

thename.substring(6,9) will return John

thename.toUpperCase will return SMITH/JOHN

thename.toLowerCase will return smith/john

• We have used the alert, there is also a confirm that shows an OK and a Cancel and saves the results as true/false: var ans = confirm("Is this correct?");
• Remember when testing a condition, is equal to is == and is not equal to is !=. Also when doing compound conditions, AND is &&, OR is || and NOT is !
• JavaScript has a switch statement that does the case structure (compare to C)
• The dowhile places the completion test at the end of the loop construct so the code in the loop is always executed at least once.
• JavaScript arrays have dynamic length.
• Array objects can be created using new or using a literal array value.

var schedule = newArray("CIS17", "CIS44", "ENG11", "MTH31");

var schedule = newArray(5);

var schedule = ["CIS53", "CIS31", "HST21"]

• Methods collection for an array

schedule.sort(); elements become string and are sorted

var rslt = schedule.join(" / "); means that the courses in the schedule are strung together with a / between them and stored in rslt.

schedule.concat() will add elements to the array

var partschedule = schedule.slice(1,2) will take out starting with element 1 through element2 (first element in array is element0)

There are others.

• Subprograms in JavaScript are functions. Note that there is a return statement which can include information returned by the function. Information can be passed to the function with parameters. Depending on the definition of data information may still be available as a result of the function even if data was not returned. When you look at functions on the web, the return is frequently not in evidence although it is the prescribed way of returning data.
• If you do not use var when declaring a variable then the variable gets implicitly declared when it is first encountered which means that it has global scope (implicitly declared variables have global scope even if the declaration is in a function). This is used frequently in the web functions you find on a variety of web sites even though programmers would prefer more formal declarations. If you explicitly declare a variable outside a function then that variable also has global scope (preferred way). A variable that is explicitly declared within a function has local scope (preferred way). Note that if a variable is defined both globally and locally, the local wins.
• The parameter passing method used by JavaScript is the pass by value method.