How do I remove objects from a javascript associative array?

All objects in JavaScript are implemented as hashtables/associative arrays. So, the following are the equivalent:

alert(myObj["SomeProperty"]);
alert(myObj.SomeProperty);

And, as already indicated, you "remove" a property from an object via the delete keyword, which you can use in two ways:

delete myObj["SomeProperty"];
delete myObj.SomeProperty;

Hope the extra info helps...

That only removes deletes the object but still keeps the array length same.

To remove you need to do something like:

array.splice(index, 1);

Problem

None of the previous answers address the fact that Javascript does not have associative arrays to begin with - there is no array type as such, see typeof.

What Javascript has, are object instances with dynamic properties. When properties are confused with elements of an Array object instance then Bad Things™ are bound to happen:

var elements = new Array()

elements.push(document.getElementsByTagName("head")[0])
elements.push(document.getElementsByTagName("title")[0])
elements["prop"] = document.getElementsByTagName("body")[0]

console.log("number of elements: ", elements.length)   // returns 2
delete elements[1]
console.log("number of elements: ", elements.length)   // returns 2 (?!)

for (var i = 0; i < elements.length; i++)
{
   // uh-oh... throws a TypeError when i == 1
   elements[i].onmouseover = function () { window.alert("Over It.")}
   console.log("success at index: ", i)
}

Solution

To have a universal removal function that does not blow up on you, use:

Object.prototype.removeItem = function (key) {
   if (!this.hasOwnProperty(key))
      return
   if (isNaN(parseInt(key)) || !(this instanceof Array))
      delete this[key]
   else
      this.splice(key, 1)
};

//
// Code sample.
//
var elements = new Array()

elements.push(document.getElementsByTagName("head")[0])
elements.push(document.getElementsByTagName("title")[0])
elements["prop"] = document.getElementsByTagName("body")[0]

console.log(elements.length)                        // returns 2
elements.removeItem("prop")
elements.removeItem(0)
console.log(elements.hasOwnProperty("prop"))        // returns false as it should
console.log(elements.length)                        // returns 1 as it should

While the accepted answer is correct, it is missing the explanation why it works.

First of all, your code should reflect the fact that this is NOT an array:

var myObject = new Object();
myObject["firstname"] = "Bob";
myObject["lastname"] = "Smith";
myObject["age"] = 25;

Note that all objects (including Arrays) can be used this way. However, do not expect for standard JS array functions (pop, push,...) to work on objects!

As said in accepted answer, you can then use delete to remove the entries from objects:

delete myObject["lastname"]

You should decide which route you wish to take - either use objects (associative arrays / dictionaries) or use arrays (maps). Never mix the two of them.

You are using Object, you are not having an associative array to begin with. With an associative array, adding and removing items goes like this:

    Array.prototype.contains = function(obj) 
    {
        var i = this.length;
        while (i--) 
        {
            if (this[i] === obj) 
            {
                return true;
            }
        }
        return false;
    }


    Array.prototype.add = function(key, value) 
    {
        if(this.contains(key))
            this[key] = value;
        else
        {
            this.push(key);
            this[key] = value;
        }
    }


    Array.prototype.remove = function(key) 
    {
        for(var i = 0; i < this.length; ++i)
        {
            if(this[i] == key)
            {
                this.splice(i, 1);
                return;
            }
        }
    }



    // Read a page's GET URL variables and return them as an associative array.
    function getUrlVars()
    {
        var vars = [], hash;
        var hashes = window.location.href.slice(window.location.href.indexOf('?') + 1).split('&');

        for(var i = 0; i < hashes.length; i++)
        {
            hash = hashes[i].split('=');
            vars.push(hash[0]);
            vars[hash[0]] = hash[1];
        }

        return vars;
    }



    function ForwardAndHideVariables() {
        var dictParameters = getUrlVars();

        dictParameters.add("mno", "pqr");
        dictParameters.add("mno", "stfu");

        dictParameters.remove("mno");



        for(var i = 0; i < dictParameters.length; i++)
        {
            var key = dictParameters[i];
            var value = dictParameters[key];
            alert(key + "=" + value);
        }
        // And now forward with HTTP-POST
                    aa_post_to_url("Default.aspx", dictParameters);
    }


    function aa_post_to_url(path, params, method) {
        method = method || "post";

        var form = document.createElement("form");

        //move the submit function to another variable
        //so that it doesn't get written over if a parameter name is 'submit'
        form._submit_function_ = form.submit;

        form.setAttribute("method", method);
        form.setAttribute("action", path);

        for(var i = 0; i < params.length; i++)
        {
            var key = params[i];

            var hiddenField = document.createElement("input");
            hiddenField.setAttribute("type", "hidden");
            hiddenField.setAttribute("name", key);
            hiddenField.setAttribute("value", params[key]);

            form.appendChild(hiddenField);
        }

        document.body.appendChild(form);
        form._submit_function_(); //call the renamed function
    }

If your intention is just to remove a single item from an existing array and you are using jQuery, I would use $.grep() method. Pass in the array you wish to remove an entry from and run an anonymous function that returns the array without the specified entry. I found this very helpful... Hope it helps!

     for(var i = 0; i < yourArray.length; i++) {

        if (yourVar == yourArray[i]) {

            yourArray= $.grep(yourArray, function (item, index) {

                return item != yourArray[i];
                //index param is not used in this instance
            });
        }
        alert("Updated Array = " + yourArray); //Display updated Array
    }

Here is the link to the jQuery API page for $.grep: http://api.jquery.com/jQuery.grep/