Functions and Methods Function : Block of instructions called (executed) by name Method: A function...
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Transcript of Functions and Methods Function : Block of instructions called (executed) by name Method: A function...
![Page 1: Functions and Methods Function : Block of instructions called (executed) by name Method: A function operating within an object –World Example: start car.](https://reader036.fdocuments.in/reader036/viewer/2022062516/56649d425503460f94a1ce8b/html5/thumbnails/1.jpg)
Functions and Methods
• Function: Block of instructions called (executed) by name
• Method: A function operating within an object– World Example: start car – object is car, method is start
– World Example: turn on light - object is light, method is turn on
• Built-in methods provided by browsers: – alert is a method in the Window object (the default, so
dot notation not required)
– write and getElementById are methods in the browser’s document object
• User-defined functions: you can write your own
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Syntax for user-defined functionsfunction yourName(param, param, …, param){ /*** JavaScript instructions go here *****/ }
Notes
• function is a reserved word that must be present
• yourName is an identifier that you choose as the name of the function
• param, param, … , param is a list of parameters (variables) that the function uses in its instructions separated by commas
• Put the block of JavaScript instructions between the braces
• To call the function: yourName(arg, arg, … ,arg); where arg, arg, … , arg are arguments (which are expressions) passed to the function so it can do its thing.
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Green Box Diagram
AFunction
orMethodcontains
ablock
ofinstructions
We pass (give) the function(or method) information (expressions)as arguments
to use
Processing is done in the green box
Out can come a value that is
returned
Variables canbe set and HTMLpages can change
asside effects
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Parameters and Arguments
• Parameter: name a function gives an expression passed to it
• Argument: expression passed to a function
• Why the difference?– We might want to use the
function more than once.
– We might want to call a function in different ways.
30function triple(x){ return x*3; }x (parameter)
triple(10);
First Call
60function triple(x){ return x*3; }x (parameter)
triple(20);
Second Call
60function triple(x){ return x*3; }x (parameter)
var z = 20;triple(z)
Third Call
90function triple(x){ return x*3; }x (parameter)
triple(triple(10));
Fourth Call
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Creating and Calling Functions
• Create a summing function:
function sum( x, y, z) { return x+y+z; }
• Call the sum function when the user clicks on a button
<input type="button" onclick="alert(sum(3,5,7));" />
• Result: 15 displays in an alert box when you click the button
• Notes– Arguments: 3, 5, and 7 (expressions passed to the function). – Parameters: x, y, z. Inside the function, x=3, y=5, and z = 7.– Parentheses enclose parameters and arguments and they are separated with
commas– Braces enclose the function's block of instructions– We call the function by using its name.
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Browser Built-in Methods• alert("hello");
– Object: window– Method Name: alert – Argument: "hello"– Returned: nothing in this case– Side affect: dialog box created
• document.write("hello");– Object: Document– Method Name: write– Argument: "hello"– Returned: nothing in this case– Side affect: data written into the web-page
Note: We can use dot notation and write window.alert (but don’t have to because window is the default object)
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Example with Parameters
<html><head><script type="text/javascript" >function sampleFunction(a,b,c,d, e){ alert(a); alert(b); alert(c); alert(d); alert(e);}</script></head><script type="text/javascript" > var x = 10 * 2 / 4; sampleFunction("Bill", 10, x, 20+10, "10"+"20");</script></body></html>
Question:What Displays? and Why?
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Returning Values
<script type="text/javascript"> var theName; theName = prompt ("Name Entry", "Default is Bob"); if (theName==null) theName = "Bob"; alert(theName);</script>
QuestionsHow many arguments? What is returned? What is the 'if'?What is 'null'? What variables are there? How are they named?
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Returning Values
<script type="text/javascript">function area(){ var width=parseFloat(document.getElementById("w").value); var height = parseFloat(document.getElementById("h").value); document.GetElementById("area").value = length*height; return false;}</script>
<form onsubmit="return area();">Enter width <input type="text" id="w" size="6" /><br />Enter height<input type="text" id="h" size= "6" /><br /><input type="submit" value="Submit Query" /><br /> Area = <input type="text" id="area" size="6" /></form>
Return false case tells the browser notto try to submit the form to a server
To return a value simply the keyword return followed by the value to return
Reminder: the star means multiply
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Image Rollover (No Arguments)
<html><head><!– Define the function --><script type="text/javascript"> function roll() { document.getElementById("myImage").src = "rabbit2.gif"; }</script>
<!– Call the function --></head><body><img id="myImage" src="rabbit.gif" /></body></html><form><input type="button" value="change it" onclick="roll();" /></form></body></html>
Question: How could you change the size or alt attributes?
Question: Do you understand how this works?
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More General Rollover with Parameters<html><head><script type="text/javascript"> function rollOver(idName, srcName) { document.getElementById(idName).src = srcName; }</script>
</head><body><img id="myImage" src="rabbit.gif" /><form><input type="button" value="change image" onmousedown= "rollOver('myImage', 'rabbit2.gif');" onmouseup = "rollOver('myImage', 'rabbit.gif');" /></form></body></html>
Question: How could you change the size of the imag?Question: Why the single quotes in the function call?
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More Information
• Check the appendix in the text book• The text also has lots of examples• http://www.w3schools.com/jsref/default.asp• http://www.wowarea.com/english/help/jsmeth.htm• http://www.c-point.com/javascript_tutorial/
objects.htm• http://jennifermadden.com/javascript/
objectsPropertiesMethods.html
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Review Questions
• What is a variable? What is a type?• What is the difference between ++x and x++?• What gets done first, multiplication or addition?• What is an object?• What is the difference between a method and a property?• When would you use the getElementById method?• Give an example of when a function might return a value.• What is the difference between a function and a method?• Why do parameters often have different names than
arguments?• What is the '{' character used for? • Give three examples of attributes connected with mouse
operations.