test/mjsunit/object-prevent-extensions.js

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// Tests the Object.preventExtensions method - ES 15.2.3.10


var obj1 = {};
// Extensible defaults to true.
assertTrue(Object.isExtensible(obj1));
Object.preventExtensions(obj1);

// Make sure the is_extensible flag is set. 
assertFalse(Object.isExtensible(obj1));
obj1.x = 42;
assertEquals(undefined, obj1.x);

// Try adding a new element.
obj1[1] = 42;
assertEquals(undefined, obj1[1]);


// Try when the object has an existing property.
var obj2 = {};
assertTrue(Object.isExtensible(obj2));
obj2.x = 42;
assertEquals(42, obj2.x);
assertTrue(Object.isExtensible(obj2));

Object.preventExtensions(obj2);
assertEquals(42, obj2.x);

obj2.y = 42;
// obj2.y should still be undefined.
assertEquals(undefined, obj2.y);
// Make sure we can still write values to obj.x.
obj2.x = 43;
assertEquals(43, obj2.x)

obj2.y = new function() { return 42; };
// obj2.y should still be undefined.
assertEquals(undefined, obj2.y);
assertEquals(43, obj2.x)

try {
  Object.defineProperty(obj2, "y", {value: 42});
} catch (e) {
  assertTrue(/object is not extensible/.test(e));
}

// obj2.y should still be undefined.
assertEquals(undefined, obj2.y);
assertEquals(43, obj2.x);

obj2[1] = 42;
assertEquals(undefined, obj2[1]);

var arr = new Array();
arr[1] = 10;

Object.preventExtensions(arr);

arr[2] = 42;
assertEquals(10, arr[1]);

// We should still be able to change exiting elements.
arr[1]= 42;
assertEquals(42, arr[1]);


// Test the the extensible flag is not inherited.
var parent = {};
parent.x = 42;
Object.preventExtensions(parent);

var child = Object.create(parent);

// We should be able to add new properties to the child object.
child.y = 42;

// This should have no influence on the parent class.
parent.y = 29;


// Test that attributes on functions are also handled correctly.
function foo() {
  return 42;
}

Object.preventExtensions(foo);

foo.x = 29;
assertEquals(undefined, foo.x);