JavaScript syntax
part 26/31 · 107.4 KB total
──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
}
this.m1 = px;
return this;
}
const foo1 = new Foo(1);
const foo2 = new Foo(0);
foo2.prefix = "b-";
console.log("foo1/2 " + foo1.pyz() + foo2.pyz());
// foo1/2 a-Y b-Z
foo1.m3 = px; // Assigns the function itself, not its evaluated result, i.e. not px()
const baz = {"prefix": "c-"};
baz.m4 = px; // No need for a constructor to make an object.
console.log("m1/m3/m4 " + foo1.m1() + foo1.m3() + baz.m4());
// m1/m3/m4 a-X a-X c-X
foo1.m2(); // Throws an exception, because foo1.m2 does not exist.
Constructors
Constructor functions simply assign values to slots of a newly created object. The values may be data or other functions.
Example: Manipulating an object:
function MyObject(attributeA, attributeB) {
this.attributeA = attributeA;
this.attributeB = attributeB;
}
MyObject.staticC = "blue"; // On MyObject Function, not object
console.log(MyObject.staticC); // blue
const object = new MyObject('red', 1000);
console.log(object.attributeA); // red
console.log(object.attributeB); // 1000
console.log(object.staticC); // undefined
object.attributeC = new Date(); // add a new property
delete object.attributeB; // remove a property of object
console.log(object.attributeB); // undefined
The constructor itself is referenced in the object's prototype's constructor slot. So,
function Foo() {}
// Use of 'new' sets prototype slots (for example,
// x = new Foo() would set x's prototype to Foo.prototype,
// and Foo.prototype has a constructor slot pointing back to Foo).
const x = new Foo();
// The above is almost equivalent to
const y = {};
y.constructor = Foo;
y.constructor();
// Except
x.constructor == y.constructor; // true
x instanceof Foo; // true
y instanceof Foo; // false
// y's prototype is Object.prototype, not
// Foo.prototype, since it was initialized with
// {} instead of new Foo.
// Even though Foo is set to y's constructor slot,
// this is ignored by instanceof - only y's prototype's
// constructor slot is considered.
Functions are objects themselves, which can be used to produce an effect similar to "static properties" (using C++/Java terminology) as shown below. (The function object also has a special prototype property, as discussed in the "Inheritance" section below.)
Object deletion is rarely used as the scripting engine will garbage collect objects that are no longer being referenced.
Inheritance
JavaScript supports inheritance hierarchies through prototyping in the manner of Self.
In the following example, the Derived class inherits from the Base class. When d is created as Derived, the reference to the base instance of Base is copied to d.base.
Derive does not contain a value for aBaseFunction, so it is retrieved from aBaseFunction when aBaseFunction is accessed. This is made clear by changing the value of base.aBaseFunction, which is reflected in the value of d.aBaseFunction.
──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────