Adding custom properties to a function

by

First of all, it’s important to realise that standard function properties (arguments, name, caller & length) cannot be overwritten. So, forget about adding a property with that name.

Adding your own custom properties to a function can be done in different ways that should work in every browser.

Adding your own custom properties to a function

Way 1 : adding properties while running the function :

var doSomething = function() {
    doSomething.name="Tom";
    doSomething.name2 = 'John';
    return 'Beep';
};

console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);
console.log('doSomething() : ' + doSomething());
console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);

Output :

doSomething.name : 
doSomething.name2 : undefined
doSomething() : Beep
doSomething.name : 
doSomething.name2 : John

Way 1 (alternate syntax) :

function doSomething() {
    doSomething.name="Tom";
    doSomething.name2 = 'John';
    return 'Beep';
};

console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);
console.log('doSomething() : ' + doSomething());
console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);

Output :

doSomething.name : doSomething
doSomething.name2 : undefined
doSomething() : Beep
doSomething.name : doSomething
doSomething.name2 : John

Way 1 (second alternate syntax) :

var doSomething = function f() {
    f.name="Tom";
    f.name2 = 'John';
    return 'Beep';
};

console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);
console.log('doSomething() : ' + doSomething());
console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);

Output :

doSomething.name : f
doSomething.name2 : undefined
doSomething() : Beep
doSomething.name : f
doSomething.name2 : John

A problem with this strategy is that you need to run your function at least once to assign the properties. For many functions, that’s obviously not what you want. So let’s consider the other options.

Way 2 : adding properties after defining the function :

function doSomething() {
    return 'Beep';
};
    
doSomething.name="Tom";
doSomething.name2 = 'John';

console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);
console.log('doSomething() : ' + doSomething());
console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);

Output :

doSomething.name : doSomething
doSomething.name2 : John
doSomething() : Beep
doSomething.name : doSomething
doSomething.name2 : John

Now, you don’t need to run your function first before you’re able to access your properties. However, a disadvantage is that your properties feel disconnected from your function.

Way 3 : wrap your function in anonymous function :

var doSomething = (function(args) {
    var f = function() {
        return 'Beep';
    };
    for (i in args) {
        f[i] = args[i];
    }
    return f;
}({
    'name': 'Tom',
    'name2': 'John'
}));

console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);
console.log('doSomething() : ' + doSomething());
console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);

Output :

doSomething.name : 
doSomething.name2 : John
doSomething() : Beep
doSomething.name : 
doSomething.name2 : John

Wrapping your function in an anonymous function, you can collect your attributes into an object and use a loop to add those attributes one-by-one within the anonymous function. That way, your attributes feel more connected to your function. This technique is also very useful for when your attributes need to be copied from an existing object. A disadvantage, however, is that you can only add multiple attributes at the same time when you define your function. Also, it doesn’t exactly result in DRY code if adding properties to a function is something you want to do often.

Way 4 : add an ‘extend’ function to your function, that adds the properties of an object to itself one by one :

var doSomething = function() {
    return 'Beep';
};
    
doSomething.extend = function(args) {
    for (i in args) {
        this[i] = args[i];
    }
    return this;
}

doSomething.extend({
    'name': 'Tom',
    'name2': 'John'
});

console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);
console.log('doSomething() : ' + doSomething());
console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);

Output :

doSomething.name : 
doSomething.name2 : John
doSomething() : Beep
doSomething.name : 
doSomething.name2 : John

This way, you can extend multiple properties and/or copy properties from another project at any time. Again, however, your code isn’t DRY if this is something you do more often.

Way 5 : Make a generic ‘extend’ function :

var extend = function(obj, args) {
    if (Array.isArray(args) || (args !== null && typeof args === 'object')) {
        for (i in args) {
            obj[i] = args[i];
        }
    }
    return obj;
}
    
var doSomething = extend(
    function() {
        return 'Beep';
    }, {
        'name': 'Tom',
        'name2': 'John'
    }
);

console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);
console.log('doSomething() : ' + doSomething());
console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);

Output :

doSomething.name : 
doSomething.name2 : John
doSomething() : Beep
doSomething.name : 
doSomething.name2 : John

A genetic extend function allows for a more DRY approach, allowing you to add the object or any project to any other object.

Way 6 : Create an extendableFunction object and use it to attach an extend function to a function :

var extendableFunction = (function() {
    var extend = function(args) {
        if (Array.isArray(args) || (args !== null && typeof args === 'object')) {
            for (i in args) {
                this[i] = args[i];
            }
        }
        return this;
    };
    var ef = function(v, obj) {
        v.extend = extend;
        return v.extend(obj);
    };

    ef.create = function(v, args) {
        return new this(v, args);
    };
    return ef;
})();

var doSomething = extendableFunction.create(
    function() {
        return 'Beep';
    }, {
        'name': 'Tom',
        'name2': 'John'
    }
);

console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);
console.log('doSomething() : ' + doSomething());
console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);

Output :

doSomething.name : 
doSomething.name2 : John
doSomething() : Beep
doSomething.name : 
doSomething.name2 : John

Rather than using a generic ‘extend’ function, this technique allows you to generate functions that have an ‘extend’ method attached to it.

Way 7 : Add an ‘extend’ function to the Function prototype :

Function.prototype.extend = function(args) {
    if (Array.isArray(args) || (args !== null && typeof args === 'object')) {
        for (i in args) {
            this[i] = args[i];
        }
    }
    return this;
};

var doSomething = function() {
    return 'Beep';
}.extend({
    name : 'Tom',
    name2 : 'John'
});

console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);
console.log('doSomething() : ' + doSomething());
console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);

Output :

doSomething.name : 
doSomething.name2 : John
doSomething() : Beep
doSomething.name : 
doSomething.name2 : John

A great advantage to this technique is that it makes adding new properties to a function very easy and DRY as well as completely OO. Also, it’s pretty memory friendly. A downside, however, is that it’s not very future proof. In case future browsers ever add a native ‘extend’ function to the Function prototype, this that could break your code.

Way 8 : Run a function recursively once and then return it :

var doSomething = (function f(arg1) {
    if(f.name2 === undefined) {
        f.name="Tom";
        f.name2 = 'John';
        f.extend = function(args) {
            if (Array.isArray(args) || (args !== null && typeof args === 'object')) {
                for (i in args) {
                    this[i] = args[i];
                }
            }
            return this;
        };
        return f;
    } else {
        return 'Beep';
    }
})();

console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);
console.log('doSomething() : ' + doSomething());
console.log('doSomething.name : ' + doSomething.name);
console.log('doSomething.name2 : ' + doSomething.name2);

Output :

doSomething.name : f
doSomething.name2 : John
doSomething() : Beep
doSomething.name : f
doSomething.name2 : John

Run a function once and have it test whether one of its properties is set. If not set, set the properties and return itself. If set, execute the function. If you include an ‘extend’ function as one of the properties, you can later execute that to add new properties.

Adding your own custom properties to an object

In spite of all these options, I would nevertheless recommend against adding properties to a function. It’s much better to add properties to objects!

Personally, I prefer the singleton classes with the following syntax.

var keyValueStore = (function() {
    return {
        'data' : {},
        'get' : function(key) { return keyValueStore.data[key]; },
        'set' : function(key, value) { keyValueStore.data[key] = value; },
        'delete' : function(key) { delete keyValueStore.data[key]; },
        'getLength' : function() {
            var l = 0;
            for (p in keyValueStore.data) l++;
            return l;
        }
    }
})();

An advantage to this syntax is that it allows for both public and private variables. For example, this is how you make the ‘data’ variable private :

var keyValueStore = (function() {
    var data = {};
    
    return {
        'get' : function(key) { return data[key]; },
        'set' : function(key, value) { data[key] = value; },
        'delete' : function(key) { delete data[key]; },
        'getLength' : function() {
            var l = 0;
            for (p in data) l++;
            return l;
        }
    }
})();

But you want multiple datastore instances, you say? No problem!

var keyValueStore = (function() {
    var count = -1;
    
    return (function kvs() {
        count++; 
        return {
            'data' : {},
            'create' : function() { return new kvs(); },
            'count' : function() { return count; },
            'get' : function(key) { return this.data[key]; },
            'set' : function(key, value) { this.data[key] = value; },
            'delete' : function(key) { delete this.data[key]; },
            'getLength' : function() {
                var l = 0;
                for (p in this.data) l++;
                return l;
            }
        }
    })();
})();

Finally, you can seperate the instance and singleton properties and use a prototype for the instance’s public methods. That results in the following syntax :

var keyValueStore = (function() {
    var count = 0; // Singleton private properties
        
    var kvs = function() {
        count++; // Instance private properties
        this.data = {};  // Instance public properties
    };
    
    kvs.prototype = { // Instance public properties
        'get' : function(key) { return this.data[key]; },
        'set' : function(key, value) { this.data[key] = value; },
        'delete' : function(key) { delete this.data[key]; },
        'getLength' : function() {
            var l = 0;
            for (p in this.data) l++;
            return l;
        }
    };
        
    return  { // Singleton public properties
        'create' : function() { return new kvs(); },
        'count' : function() { return count; }
    };
})();

With this syntax, you can have :

  • multiple instances of an object
  • private variables
  • class variables

You use it like this :

kvs = keyValueStore.create();
kvs.set('Tom', "Baker");
kvs.set('Daisy', "Hostess");
var profession_of_daisy = kvs.get('Daisy');
kvs.delete('Daisy');
console.log(keyValueStore.count());

More Related Contents:

  1. How do I use ‘this” keyword in an object literal? [closed]
  2. How do I remove a property from a JavaScript object?
  3. Is it possible to get the non-enumerable inherited property names of an object?
  4. How does the new operator work in JavaScript?
  5. How to add an object to an array
  6. `new function()` with lower case “f” in JavaScript
  7. Clone Object without reference javascript [duplicate]
  8. Extending an Object in Javascript
  9. javascript : function and object…?
  10. Why in JavaScript both “Object instanceof Function” and “Function instanceof Object” return true?
  11. JavaScript Extending Class
  12. Function and Object Javascript
  13. Multiple arguments vs. options object
  14. Find all values by specific key in a deep nested object
  15. Get values from an object in JavaScript [duplicate]
  16. Can you alter a Javascript function after declaring it?
  17. What does the new keyword do under the hood?
  18. What is wrong with this function
  19. Most efficient method to groupby on an array of objects
  20. Convert Array to Object
  21. How is almost everything in Javascript an object?
  22. Sorting a JavaScript object by property name
  23. How to define custom sort function in javascript?
  24. JS: Does Object.assign() create deep copy or shallow copy
  25. Is there a JavaScript function that reduces a fraction
  26. Using JavaScript what’s the quickest way to recursively remove properties and values from an object?
  27. Javascript – deepEqual Comparison
  28. Why is “forEach not a function” for this object?
  29. Passing a local variable from one function to another
  30. Function not calling within an onclick event

Leave a Comment