Introduction to JavaScript for Python Programmers

Lecture Outline How Python & JavaScript are similar How Python & Javascript are different JavaScript fundamentals JavaScript Examples

Python & Javascript: Similarities Both are programming languages Used by programmers to tell the computer what to do. Both are interpreted languages JavaScript is interpreted by the web browser, Python is interpreted by IDLE or the python interpreter. Both used to implement algorithms Series of actions and calculations, that use decisions and loops to control the flow of execution. Both are imperative languages

Python & Javascript: Differences Different web browsers support different JavaScript code. The basic syntax works on all browsers, but some browsers support non-standard extensions that are not supported on other browsers. The source code can be stored inside of a web page, sometimes intermixed with HTML. Web browsers typically do not provide good debugging information about why a JavaScript fails, so debugging a non-working script can be difficult. The Syntax of Python & Javascript are different Statements can be terminated with semi-colons. Brackets must be used to indicate blocks.

Javascript in a Web Page JavaScript is separated from HTML by putting it inside of special script tags: <script laguage=javascript> JAVASCRIPT JAVASCRIPT </script> Some javascript is actually part of HTML, for example, a function call on a button press: onClick=”cwCalc();” Output from a JavaScript program usually is shown to the user as HTML that is rendered by the web browser.

Javascript: Syntax Differences Indentation does NOT indicate blocks. Instead, curly brackets are used to indicate blocks. (Like C and C++)‏ Python: def myFunc(aVar): if (aVar == “test”): return( 0) else: return(-1) JavaScript: function myFunc(aVar) { if (aVar == 'test') { return(0) } else { return (-1) } }

Javascript: Indentation does NOT matter! This: function myFunc(aVar) { if (aVar == 'test') { return(0) } else { return (-1) } } functions the same as this: function myFunc(aVar) { if (aVar == 'test') { return(0) } else { return (-1) } }

But please use good indentation! Improves readability and understandability function myFunc(aVar) { if (aVar == 'test') { return(0) } else { return (-1) } }

Calling Functions, & Returning Values The previous example should make it clear that calling functions and returning values works basically the same in JavaScript as it does in Python. function myFunc(aVar) { if (aVar == 'test') { return(0) } else { return (-1) } } myfunc(“aString”)‏

Declaring Variables and Arrays Variables must be declared before being used: var VARIABLENAME = VALUE var myName = “Jay” The “VAR” keyword tells the JavaScript interpreter that you are making a variable. Arrays must have their size declared at time of creation: var ARRAYNAME = new Array(SIZE) var myArray = new Array(50) You index elements in an array using bracket notation: var oneElement = myArray[1]

Conditional (IF) Statements Very similar in Python & JavaScript. IF and IF/ELSE are the same (except for brackets). No ELIF statement. “Weird Conditionals”: I won't talk about them, if you run into them, do a quick web search to figure out how they work. Switch/case/break ?:

Looping (while) statement Very similar in Python & JavaScript. while (BOOLEAN EXPRESSION) { STATEMENT; STATEMENT; } New do/while statement. Test is performed at the end of the loop instead of the beginning: do { STATEMENT; STATEMENT; } while (BOOLEAN EXPRESSION)

Looping (for) statement FOR loops in JavaScript are based on C and C++ syntax, and are usually quite different than in Python. for( INITIALIZE; BOOLEAN EXPRESSION; ACTION) { STATEMENT; STATEMENT; } for (var j = 0; j < 5; j = j+1) { document.write(“j is: “ + j) }

Looping (for) statement FOR loops in JavaScript are based on C and C++ syntax, and are usually quite different than in Python. for( INITIALIZE; BOOLEAN EXPRESSION; ACTION) { STATEMENT; } Code before the first semicolon (the initialization code) is executed once at the beginning of the loop. The boolean expression (in the middle) is evaluated every time the loop repeats, and the statements in the loop are only executed if the result is TRUE. The action code is executed after the loop executes, and is typically used to increment a counter.

Looping (for) Statement Example for (var j = 0; j < 5; j = j+1) { document.write(“j is: “ + j) } The document.write statement executes 5 times, with j equalling 0,1,2,3, and 4. After the 5th execution, the j=j+1 action executes, and then the j < 5 test evaluates to FALSE, so document.write() does not execute when j = 5.

Looping (for) Statement – Iteration over Properties JavaScript does have a FOR/IN version of the for loop, but it doesn't work exactly as you'd expect from your experience with Python's FOR loops. for (VARNAME in OBJECT) { document.write( VARNAME ) document.write( OBJECT[VARNAME] ) } Note that it iterates through the property names of an object, and you have to use bracket notation to get the property values.

Input / Output with JavaScript As most JavaScript runs inside of a web browser, most of the input and output to and from a JavaScript program goes through the web-browser. Input comes from the user via web forms, button and link clicks. Output is typically is displayed as a webpage or part of a webpage as HTML rendered by the web browser.

Webpage Output with JavaScript A script can use the document.write( “a string”) function to add content to a web page as it is loaded. <html> <body> <h1>Example:</h1> <script language=javascript> document.write(“<h2>Here is my website!</h2>”) </script> </body> </html> Note that this javascript code will be executed when it is encountered, and it outputs HTML that is rendered by the web browser.

Webpage Output with JavaScript A script can output calculated and dynamic information: <html> <body> <h1>Clock Example:</h1> <script language=javascript> var date = new Date() var hours = date.getHours() var min = date.getMinutes() document.write(“Time: <b>” +hours+”:”+min+”</b>”) </script> </body> </html>

Dialog Output with JavaScript A script can use the alert( “a string”) function to pop up an “Alert” Dialog box. <html> <body> <h1>Example:</h1> <script language=javascript> alert(“Here is an annoying pop-up!”) </script> </body> </html> This javascript code will be executed when it is encountered, and causes an alert-box to appear.

Input: Running a function when the user clicks Define a function: <script language=javascript> function myFunc() { alert(“You clicked it!”) } </script> You can use an onClick=”FUNCTIONNAME();” parameter on a button to call the function: <input type=button value=”Do It!” onClick=”myFunc();” > Clicking on the button will cause the alert box to appear.

Input: Running a function when the user clicks <html> <body> <script language=javascript> function myFunc() { alert("You clicked it!")‏ } </script> <input type=button value="Do It!" onClick="myFunc();" > </body> </html> Input: Running a function when the user clicks

Output: Modifying an already loaded webpage document.write() can be used to add text to a webpage as it is being loaded. To modify text in a webpage AFTER it is loaded, you can use a named area (DIV) and replace the HTML content with javascript. <div id="txt">Plain Text</div> is HTML that creates a DIV named “txt”. You can then change the HTML content of this DIV (currently “Plain Text”) with javascript as follows: var myDIV = document.getElementById('txt') myDIV.innerHTML=”<b>New Text!</b>”

Putting it all together: An example program <html> <body> <script type="text/javascript"> var num = 0 function startTime() { num = num + 1 var myDiv = document.getElementById('txt')‏ myDiv.innerHTML="<b>Clicked " +num+ " Times</b>"; } </script> <div id="txt">Plain Text</div> <input type=button value="click it!" onClick="startTime()"> </body> </html>