Writing Scripts in Nashorn - Scripting in Java: Integrating with Groovy and JavaScript - page 103

In this code, you could have used the defineProperties() function to define both x
and y properties in one shot. Both functions defineProperty() and defineProperties()
return the object on which the property or properties are being set so that you can chain
their calls. The following code rewrites the previous example:

// Create an empty object
var origin2D = {};

// Add two non-writable x and y properties to origina2D
// with their value set to 0.
Object.defineProperties(origin2D, {x: {value:0, writable: false},
y: {value:0, writable: false}});

■ When you add a property to an object in the object literal or by assigning a value
to the property, the default values for writable , enumerable , and configurable properties
are set to true . When you use a property descriptor to define a property or alter the property's
attributes, the default values for these attributes in the property descriptor is false .
Make sure you specify values for all attributes that needs to be set to true when using
a property descriptor.

Tip

Once you set the writable attribute of a property to false , changing its value has no
effect. In strict mode, changing the value of a nonwritable property generates an error:

var point = {x:0, y:10};
printf("x = %d", point.x);

// Make x non-writable
Object.defineProperty(point, "x", {writable: false});

// Try changing the value of x
point.x = 100; // Has no effect, because x is non-writable

printf("x = %d", point.x);

x = 0
x = 0