[书摘]private properties with closures

 

// From JavaScript: the definitive guide, 5th edition



// This function adds property accessor methods for a property with



// the specified name to the object o. The methods are named get<name>



// and set<name>. If a predicate function is supplied, the setter



// method uses it to test its argument for validity before storing it.



// If the predicate returns false, the setter method throws an exception.



//// The unusual thing about this function is that the property value



// that is manipulated by the getter and setter methods is not stored in



// the object o. Instead, the value is stored only in a local variable



// in this function. The getter and setter methods are also defined



// locally to this function and therefore have access to this local variable.



// Note that the value is private to the two accessor methods, and it cannot



// be set or modified except through the setter.

function makeProperty(o, name, predicate) {

var value; // This is the property value

// The setter method simply returns the value.

o["get" + name] = function() { return value; };

// The getter method stores the value or throws an exception if

// the predicate rejects the value.

o["set" + name] = function(v) {

if (predicate && !predicate(v))

throw "set" + name + ": invalid value " + v;

else

value = v;

};

}

// The following code demonstrates the makeProperty() method.

var o = {}; // Here is an empty object

// Add property accessor methods getName and setName()

// Ensure that only string values are allowed

makeProperty(o, "Name", function(x) { return typeof x == "string"; });

o.setName("Frank"); // Set the property value

print(o.getName()); // Get the property value

o.setName(0); // Try to set a value of the wrong type