How do I remove objects from a JavaScript associative array?

Objects in JavaScript can be thought of as associative arrays, mapping keys (properties) to values.

To remove a property from an object in JavaScript you use the delete operator:

const o = { lastName: 'foo' }
o.hasOwnProperty('lastName') // true
delete o['lastName']
o.hasOwnProperty('lastName') // false

Note that when delete is applied to an index property of an Array, you will create a sparsely populated array (ie. an array with a missing index).

When working with instances of Array, if you do not want to create a sparsely populated array - and you usually don't - then you should use Array#splice or Array#pop.

Note that the delete operator in JavaScript does not directly free memory. Its purpose is to remove properties from objects. Of course, if a property being deleted holds the only remaining reference to an object o, then o will subsequently be garbage collected in the normal way.

Using the delete operator can affect JavaScript engines' ability to optimise code.

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...

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:

Problem

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 JavaScript array functions (pop, push, etc.) 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.

There is an elegant way in the Airbnb Style Guide to do this (ECMAScript 7):

const myObject = {
  a: 1,
  b: 2,
  c: 3
};
const { a, ...noA } = myObject;
console.log(noA); // => { b: 2, c: 3 }

Copyright: https://codeburst.io/use-es2015-object-rest-operator-to-omit-properties-38a3ecffe90

You can do the following if you want a more functional and elegant approach:

const o = { firstName: "foo", lastName: "bar" };
const { lastName, ...removed } = o;
lastName // bar
removed // { firstName: "foo" }

Note that the value of removed will be undefined if there are no items left in the object.

For "Arrays":

If you know the index:

array.splice(index, 1);

If you know the value:

function removeItem(array, value) {
    var index = array.indexOf(value);
    if (index > -1) {
        array.splice(index, 1);
    }
    return array;
}

The most upvoted answer for delete works well in case of objects but not for the real arrays. If I use delete it removes elements from loops but keeps the element as empty and length of array wont change. This may be a problem in some scenarios.

For example, if I do myArray.toString() on myArray after removal via delete, it creates an empty entry, i.e. ,,.

The only working method for me:

function removeItem (array, value) {
    var i = 0;
    while (i < array.length) {
        if(array[i] === value) {
            array.splice(i, 1);
        } else {
            ++i;
        }
    }
    return array;
}

Usage:

var new = removeItem( ["apple","banana", "orange"],  "apple");
// ---> ["banana", "orange"]