DevGuru JavaScript OBJECT: Function



			OBJECT: Function new Function([arg1[, arg2[, ... argN]],] functionBody) The Function object permits a function to have methods and properties associated with it. To accomplish this, the function is temporarily considered to an object whenever you wish to invoke a method or read the value of a property. Note that JavaScript treats the function, itself, as a data type that has a value. To return that value, the function must have a return statement. When a Function object is created by using the Function constructor, it is evaluated each time. This is not as efficient as the alternative method of declaring a function using the function statement where the code is compiled. The following example creates a Function object to calculate the average of two numbers, and then displays that average: Code: var twoNumAverage = new Function("x", "y", "return (x + y)/2") document.write(twoNumAverage(3,7)) The Function object can be called just as if it were a function by specifying the variable name: Code: var average = twoNumAverage(12,17) If a function changes the value of a parameter, this change is not reflected globally or in the calling function, unless that parameter is an object, in which case any changes made to any if its properties will be reflected outside of it. In the next example, an object called TaxiCo is created with a property containing the size of the taxi fleet. A Function object called AddCar is then created which allows a user to alter the size of the Fleet property to reflect an increase in the number of cars, and then an instance of this function adds 2 to the Fleet property with the final line of code displaying the new size of the taxi fleet: Code: taxiCo = {name:"City Cabs", phone:"321765", fleet:17} var addCar = new Function("obj", "x", "obj.fleet = obj.fleet + x") addCar(taxiCo, 2) document.write("New fleet size = " + taxiCo.fleet) Output: New fleet size = 19 The function in the above example could also be created by declaring it using the function statement as follows... function addCar(obj,x){obj.fleet = obj.fleet + x} ...the difference being that when you use the Function constructor, AddCar is a variable whose value is just a reference to the function created, whereas with the function statement AddCar is not a variable at all but the name of the function itself. You can also nest a function within a function in which case the inner function can only be accessed by statements in the outer function. The inner function can use arguments and variables of the outer function, but not vice versa. The following example has an inner function that converts a monetary value from Pounds Sterling into Dollars. The outer function takes four values (the first two in Dollars and the second two in Pounds), passes each of the Pound values to the inner function to be converted to Dollars, and then adds them all together returning the sum: Code: function totalDollars(v,w,x,y) { function convertPounds(a) { return a * 1.62 } return v + w + convertPounds(x) + convertPounds(y) } document.write("Total Dollars = " + totalDollars(400, 560, 250, 460)) Output: Total Dollars = 2110.2 A Function object can also be assigned to an event handler (which must be spelled in lowercase) as in the following example: Code: window.onmouseover = new Function("document.bgColor='lightgreen'") A Function object can be assigned to a variable, which in turn can then be assigned to an event handler, provided it doesn't take any arguments, because event-handlers cannot handle them. In the following example a function which changes the background color to green is assigned to a variable ChangeBGColor. This in turn is assigned to an onMouseOver event connected to an anchor in a line of text: Code: <script language="javascript"> var changeBGColor = new Function("document.bgColor='lightgreen'") </script> He turned <a name="ChangeColor" onMouseOver="changeBGColor()">green </a> with envy. PROPERTIES arguments Property The arguments property consists of an array of all the arguments passed to a function. Syntax: [Function.]arguments arguments.callee Property The arguments.callee property can only be used within the body of a function and returns a string specifying what that function is. Syntax: [Function.]arguments.callee arguments.caller Property The arguments.caller property is deprecated in JavaScript 1.3 and is no longer used, but where it is, it specifies the name of the function that called the currently executing function. Syntax: [Function.]arguments.caller arguments.length Property The arguments.length property returns the number of arguments passed to a function. Syntax: [Function.]arguments.length arity Property The arity property specifies the number of arguments expected by a function. It is external to the function and is in contrast to the arguments.length property which specifies the number of arguments actually passed to a function. Syntax: [Function.]arity Compare the length property below. constructor Property The constructor property specifies the function that creates an object's prototype. It is a direct reference to the function itself rather than a string containing the function's name. See the constructor property of the Object object for more details and examples. Syntax: Function.constructor length Property The length property specifies the number of arguments expected by a function. It is external to the function and is in contrast to the arguments.length property which specifies the number of arguments actually passed to a function. Compare the arity property above. Syntax: Function.length prototype Property The prototype property is a value from which all instances of an object are constructed, and which also allows you to add other properties and methods to an object. See also the prototype property of the Object object for more details and examples. Syntax: Function.prototype.name = value METHODS apply Method The apply method allows you to apply to a function a method from another function. Syntax: Function.apply(thisArg[, argArray]) call Method The call method allows you to call a method from another object Syntax: Function.call(thisArg[, arg1[, arg2[, ...]]]) toSource Method The toSource method creates a string representing the source code of the function. This over-rides the Object.toSource method Syntax: Function.toSource () toString Method The toString method (like the valueOf method below) returns a string which represents the source code of a function. This over-rides the Object.toString method. Syntax: Function.toString () valueOf Method The valueOf method (like the ToString method above) returns a string which represents the source code of a function. This over-rides the Object.valueOf method. Syntax: Function.valueOf ()