Web Programming Step by Step

Lecture XX
More JavaScript Syntax

Except where otherwise noted, the contents of this presentation are Copyright 2010 Marty Stepp and Jessica Miller.

JavaScript in HTML body (example)

<script type="text/javascript">
	JavaScript code
</script>

• JS code can be embedded within your HTML page's head or body
• runs as the page is loading
• this is considered bad style and shouldn't be done in this course
  • mixes HTML content and JS scripts (bad)
  • can cause your page not to validate

Injecting Dynamic Text: document.write

document.write("message");

• prints specified text into the HTML page
• this is very bad style; this is how newbs program JavaScript:
  • putting JS code in the HTML file's body
  • having that code use document.write
  • (this is awful style and a poor substitute for server-side PHP programming, which we'll learn later)

The typeof function

typeof(value)

• given these declarations:
  • function foo() { alert("Hello"); }
  • var a = ["Huey", "Dewey", "Louie"];

• The following statements are true:
  • typeof(3.14) === "number"
  • typeof("hello") === "string"
  • typeof(true) === "boolean"
  • typeof(foo) === "function"
  • typeof(a) === "object"
  • typeof(null) === "object"
  • typeof(undefined) === "undefined"

The arguments array

function example() {
	for (var i = 0; i < arguments.length; i++) {
		alert(arguments[i]);
	}
}

example("how", "are", "you");   // alerts 3 times

• every function contains an array named arguments representing the parameters passed
• can loop over them, print/alert them, etc.
• allows you to write functions that accept varying numbers of parameters

The "for each" loop

for (var name in arrayOrObject) {
	do something with arrayOrObject[name];
}

• loops over every index of the array, or every property name of the object
• using this is actually discouraged, for reasons we'll see later

Arrays as maps

var map = [];
map[42] = "the answer";
map[3.14] = "pi";
map["champ"] = "suns";

• the indexes of a JS array need not be integers!
• this allows you to store mappings between an index of any type ("keys") and value
• similar to Java's Map collection or a hash table data structure

Date object

var today = new Date();               // today

var midterm = new Date(2007, 4, 4);   // May 4, 2007

• methods
  • getDate, getDay, getMonth, getFullYear, getHours, getMinutes, getSeconds, getMilliseconds, getTime, getTimezoneOffset, parse, setDate, setMonth, setFullYear, setHours, setMinutes, setSeconds, setMilliseconds, setTime, toString
• quirks
  • getYear returns a 2-digit year; use getFullYear instead
  • getDay returns day of week from 0 (Sun) through 6 (Sat)
  • getDate returns day of month from 1 to (# of days in month)
  • Date stores month from 0-11 (not from 1-12)

The eval (evil?) function

eval("JavaScript code");

eval("var x = 7; x++; alert(x / 2);");  // alerts 4

• eval treats a String as JavaScript code and runs that code
• this is occasionally useful, but usually a very bad idea
  • if the string's contents come from user input, the user can cause arbitrary code execution
  • can lead to security problems and bugs