How Property Access Works Friday, 5 October 2007

It's important for web developers to understand how the property access operators work. Here's a javascript basic lesson that every web developer should know:

(function(){
var a = "A String?";

a.isString = true;

return a.isString;
})();

What should be returned? Will it be the value true?

What happened to the isString property?

To answer that question, you need to know how the property access operator works.

The property access operator converts the value of its left hand operand (if it is a primitive) to an object and gets the property off that object.

Effectively, the above example is equivalent to:

    var a = "A String?";
    Object(a).isString = true;
    return Object(a).isString; // undefined

An object is created, assigned the property isString with the value true, and then becomes inaccessible by program code.

How about assigning properties to String objects?

(function(){
var a = "Axel";
a = new String(a);

a.foo = 123;

return a.foo;
})();

Variable a is now a String object. Objects can have properties.

The same conversion does not, unfortunately, take place with the instanceof operator.

(function(){
    var a = "Al Gore";
    return a instanceof String;
})();

It is still possible to check the constructor property of the object that is created by the property access operator.

(function(){
    var a = "avatar";
    return a.constructor === String;
})();

The typeof operator returns "string" for string values and "object" for String objects.

(function(){
    var a = "apple";
    var b = new String("bork");
    return typeof a + "; " + typeof b;
})();

Primitive values are not objects. Don't let the property access operators fool you.

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