javascript

Chapter 22: The Evolution of JavaScript
and [[Flexible]] attributes on each property to false . You can determine if an object is frozen by
using the Object.isFrozen() method and passing in the object. Any objects that are frozen are also
considered to be sealed and non - extensible, so Object.isSealed() will return true and
Object.isExtensible() will return false . Consider the following example:
var person = {
name: “Nicholas”,
age: 29
};
alert(Object.isSealed(person)); //false
Object.freeze(person);
alert(Object.isFrozen(person)); //true
alert(Object.isSealed(person)); //true
alert(Object.isExtensible(person)); //false
alert(delete person.name); //false
alert(person.name); “Nicholas”
person.name = “Michael”;
alert(person.name); “Nicholas”
In this example, the person object is frozen, so there is no way to add, delete, or change any properties.
The attempt to delete person.name fails as does the attempt to change its value, which fails silently.
Freezing an object ensures that it cannot be changed again during the lifetime of the program.
Once an object is locked down in any of the three ways, it is locked down until the
object goes out of scope. You cannot programmatically undo a lock down.
Changes to Functions
ECMAScript 3.1 includes several changes to functions in order to make them easier to use and debug.
To begin, the language now natively supports function currying (as discussed in Chapter 18 ) through the
addition of a bind() method on each function. This method accepts a variable number of arguments
where the first argument becomes the value of this , and each subsequent argument becomes a bound
argument. The bind() method works in the same way as the last version of the bind() function in
Chapter 18. Here’s an example :
var person = {
name: “Nicholas”,
age: 29
};
function sayName(){
alert(this.name);
}
var boundSayName = sayName.bind(person);
boundSayName(); //”Nicholas”
This code creates a function called sayName() that references the this object. You can create a version of
the function that is bound to the variable person by calling sayName.bind(person) . The returned
function can then be called directly without any arguments. The bind() method has already been
implemented natively in Chrome and is scheduled to be included in Firefox 3.1.
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