Extracting values from JavaScript objects

Introduction

The JUnify unification library allows you to extract values from JavaScript objects, for example JSON data structures. The variable syntax is however a bit cumbersome when all you want it to extract properties from objects, because it requires you to repeat the property name twice (e.g. {title: $('title')}.) The syntax would be simpler if we could just indicate that we'd like a property extracted, for example {title: _}. We will use the visit_pattern method from the JUnify library to implement a simple object extraction module.

Implementation

We start by creating a new module called extract. In this module we define some short-hand variables to methods in the unification library. We create a new visitor object and implement the object callback function. When the object value equals the wildcard _ pattern we return a new variable with the key as name. If the value is not a wildcard we return it unmodified.

extract = function () {
    var unify         = unification.unify,
        variable      = unification.variable,
        visit_pattern = unification.visit_pattern,
        _             = unification._;

    var visitor = { 
        'object' : function(key, value) {
            if (value === _) {
                return variable(key);
            }
            else {
                return value;
            }
        }
    };

    return function (pattern, value) {
        return unify(visit_pattern(pattern, visitor), value);
    };
}();

An example of its use:

var json = {text: "Hello", name: "World!"};
var l = extract({text: _, name: _}, json); // l.text = "Hello", l.name = "World!"
var r = extract({_:_, name: _}, json);     // r.name = "World!"

Note that the simplified extract syntax only works on objects; array values will require the use of the variable method. Also, if your data contains the same property names twice, only the value of the last occurance will be returned. In this case it is better to use the normal variable syntax.

Extracting typed values

The JUnify unification library also supports matching typed values and we can also support this in our simplified syntax. The syntax we will use is: {myname: MyType} which returns a myname binding if it is matched against another object with property name myname and of type MyType. In JavaScript objects are typed according to their constructors and constructors are functions. So to create typed variable we need to test if the value is a function and if it is, return a new typed variable. Modify the visitor in the following way:

    var visitor = { 
        'object' : function(key, value) {
            if (value === _) {
                return variable(key);
            }
            else if (typeof value === 'function') {
                return variable(key, value);
            }
            else {
                return value;
            }
        }
    };

That's all we need to do to add support for extracting typed objects. An example:

var o = {date: new Date()};
var r = extract({date: Date}, o); // r.date = "Sat Jul 05 2008 (…)"

This concludes the implementation of an object property extractor using the JUnify library. You can download the code extract.js for your convenience.