var boxArea = function() {
this.width=2;
};
alert(boxArea.width);
Good day. Why does this return an undefined value?
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2Read up the on javascript's module pattern. It explains how your width variable is private to the boxArea object.Straseus– Straseus2012-03-22 23:33:50 +00:00Commented Mar 22, 2012 at 23:33
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will work with "= new function"qux– qux2012-03-22 23:36:01 +00:00Commented Mar 22, 2012 at 23:36
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2 Answers
Because you created a function with that syntax. You have to add the "new" keyword in front of the function() to make it the equivalent of a class.
jsFiddle Demo: http://jsfiddle.net/dfUQu/
var boxArea = new function() {
this.width=2;
};
alert(boxArea.width);
7 Comments
DevX
okay so the new keyword makes a new object. but isn't it that functions are also object?
Timeout
They are similar in syntax but designed for different purposes. A function is just meant to be called in order to perform some work. Once it exits anything defined inside is now out of scope and discarded. A class is meant to be instantiated and reused. The new keyword creates an instance of any user defined (or build in object, like Date() for example).
DevX
Can I say that var person = {}; is same as var person=new object();
Timeout
Yes, that is called "object literal" syntax.
DevX
and what is the equivalent syntax for "new function()"?
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The classic way to create a javascript constructor is using a declared function:
function BoxArea(width) {
this.width = width;
}
By convention, constructors have names starting with a capital letter. Then you create an instance:
var ba = new BoxArea(2);
alert(ba.width); // 2
For such a simple object you could just do:
var ba = {width: 2};
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