Develop Reference

How to check if function exists inside another function using Javascript

I`m trying to build an function that load script on-demand. That is my current code:
function loadScript(src, name = null)
{
var dfd = jQuery.Deferred();
if (name === null) {
name = src.split(/\\|\//); // split by folder separator
name = name[(name.length - 1)].split('.'); // catch last index & split by extension
name.splice(name.length - 1, 1) // Remove last
name = name.join('.'); // add points in the middle of file name
}
if ( typeof name === 'function' ) return dfd.promise().resolve(name);
$.getScript( src )
.done(function( script, textStatus ) {
dfd.resolve(name);
})
.fail(function( jqxhr, settings, exception ) {
dfd.reject(exception);
});
return dfd.promise();
}
My problem is on this part of code:
if ( typeof name === 'function' ) return dfd.promise().resolve(name);
Where name is a variable that contains desired function name to check, but not the real function name, causing function never evaluate as 'function'.
I tried:
typeof `${name}` // resulting a "string"
eval("typeof name === 'function'") // But my node system not accept eval due to a potentially security risk
How many alternatives that I have ?

You could do typeof eval(name) === function or if the function is a global, typeof window[name] === function
Demo:
(function() {
function test() {}
(function(name) {
console.log(typeof eval(name) === 'function');
})('test');
})();

Quick and dirty:
if ( typeof name === 'string' && typeof eval(name) === 'function' ) return dfd.promise().resolve(name);
since you probably want to double-check that name is actually a before passing it to eval. You probably also want to further validate it if it's coming from user input as you could be opening yourself up to script injections.

Hope this could helps anyone;
const getFunc = (s) => {
const a = s.split(".");
let obj = window, i = 0;
while (i < a.length && (obj = obj[a[i++]]) !== undefined);
if (typeof obj === "function") return obj;
};
console.log(getFunc("Infinity.isFloat"));

JS: And if every function return "this" as a default value?

I have a code design question. Consider the following code:
thatObj.doThis().setThat().add().update();
To allow chaining, I'm often writing return this;, and sometimes, here or there I forget to do it, then I got an error.
In many cases, I'm not asking for a particular result (e.g. thatObj.getName() or thatObj.getChildren()), but instead wanting to do some updates or calling setters (e.g. thatObj.setName("foo") or thatObj.addChild(child) or thatObj.update()), I was wondering if it would be more convenient to return this; for any call of methods, I mean as a javascript default behaviour, and if not, what could be the reasons not to do so.

JS returns undefined if you don't return something explicitely,
JS constructors return this unless your constructor returns an Object.
CoffeeScript returns the last expression by default,
You want this to be returned by default by all methods on an object
everybody has it's own opinion what's the "right" way to do it.
could it be a good idea to say from now JS will always return this from any methods ?
And from one moment to the other, at least 2/3 of the web will be broken. So, tell me, is it a good idea?
JS has set its rules a long time ago, and something that basic is not going to change (as Pointy already mentioned). So why don't you take care of that behaviour:
//extracted that from the function to avoid memory leakage
function wrapFunction(fn) {
return function() {
let result = fn.apply(this, arguments);
return result === undefined ? this : result;
}
}
//filter === false => only own methods
//filter === true => own methods and inherited methods
//filter is Array => only the passed keys (if they are methods)
//filter is RegExp => use the RegExp to filter the keys
//filter is function => regular filterFunction
function returnThisAsDefault(objectOrConstructor, filter = false) {
if (objectOrConstructor !== Object(objectOrConstructor))
throw new TypeError("Passed argument must be an object or a constructor. Got ", typeof objectOrConstructor);
const validKey = key => typeof proto[key] === "function" && key !== "constructor" && key !== "prototype";
let proto = typeof objectOrConstructor === "function" ?
objectOrConstructor.prototype :
objectOrConstructor;
let filterFn = Array.isArray(filter) ? filter.includes.bind(filter) :
filter === false || filter === true ? () => true :
filter instanceof RegExp ? filter.test.bind(filter) :
typeof filter === "function" ? filter :
() => false;
let wrapped = {};
for (let p = proto, done = new Set(["constructor", "prototype"]); p != null && p !== Object.prototype;) {
for (let key of Object.getOwnPropertyNames(p)) {
if (typeof proto[key] !== "function" || done.has(key) || !filterFn.call(p, key)) continue;
done.add(key);
let d = Object.getOwnPropertyDescriptor(p, key);
//typeof d.value !== "function" means that proto[key] contains a getter returning a function
if (!d.writable && !d.configurable || typeof d.value !== "function") {
console.log(`function ${JSON.stringify(key)} not fit to be wrapped`, d);
continue;
}
d.value = wrapFunction(d.value);
wrapped[key] = d;
}
if (filter === false) break;
else p = Object.getPrototypeOf(p);
}
Object.defineProperties(proto, wrapped);
return objectOrConstructor;
}
let thatObject = returnThisAsDefault({
doThis() {
console.log("doThis()");
},
setThat() {
console.log("setThat()");
},
add() {
console.log("add()");
},
update() {
console.log("update()");
return "success";
},
});
let result = thatObject.doThis().setThat().add().update();
console.log("result: ", result);
.as-console-wrapper {
top: 0;
max-height: 100%!important
}

Concept: Discern between Array, Pseudo-Array, and Object in JavaScript

Often while using JavaScript you run into silly problems. One such problems is discerning between Object types.
Is there a way to create a function with this functionality? See Below:
_discern = function () { [ function code ] };
_discern({}); // Logs: Object
_discern([]); // Logs: Array
_discern(document); // Logs: Pseudo-Object
_discern(document.querySelectorAll("*")); // Logs: Pseudo-Array
I've already tried creating a function that checks for array-likeness, but that didn't work as good as I had hoped:
isArrLike = function (_) {
_[0] = 0; return [].slice.call(_).length >= Object.values(_).length;
};
And I've tried using that behavior into another function. All failures. Is there a way?

Looks like you want to know if something behaves like an array, instead of looking for the actual type/constructor.
If that's the case, it should be enough to check if something is iterable:
function isIterable(obj) {
// checks for null and undefined
if (obj == null) {
return false;
}
return typeof obj[Symbol.iterator] === 'function';
}

All of these are objects:
is array → return x instanceof Array or Array.isArray(x)
is function → return typeof x === 'function'
is HTML element → return x instanceof HTMLElement
is plain object → return typeof x === 'object' && /* ... is not array, not function, not HTML element */

Thanks to Logain's answer, I can solve the problem. Here's my approach:
kind = function (a) {
let u = toString.call(a.valueOf()).slice(8, -1);
if (a == null || u == "String" || u == "Number" || u == "Boolean") {
return u;
}
else if (typeof a[Symbol.iterator] == "function") {
return u != "Array" ? "Array-Like" : u;
}
else {
return u;
}
};

How to check if function exists in JavaScript?

My code is
function getID( swfID ){
if(navigator.appName.indexOf("Microsoft") != -1){
me = window[swfID];
}else{
me = document[swfID];
}
}
function js_to_as( str ){
me.onChange(str);
}
However, sometimes my onChange does not load. Firebug errors with
me.onChange is not a function
I want to degrade gracefully because this is not the most important feature in my program. typeof gives the same error.
Any suggestions on how to make sure that it exists and then only execute onChange?
(None of the methods below except try catch one work)

Try something like this:
if (typeof me.onChange !== "undefined") {
// safe to use the function
}
or better yet (as per UpTheCreek upvoted comment)
if (typeof me.onChange === "function") {
// safe to use the function
}

I had this problem. if (obj && typeof obj === 'function') { ... } kept throwing a reference error if obj happened to be undefined, so in the end I did the following:
if (typeof obj !== 'undefined' && typeof obj === 'function') { ... }
However, a colleague pointed out to me that checking if it's !== 'undefined' and then === 'function' is redundant, thus:
Simpler:
if (typeof obj === 'function') { ... }
Much cleaner and works great.

Modern JavaScript to the rescue!
me.onChange?.(str)
The Optional Chaining syntax (?.) solves this
in JavaScript since ES2020
in Typescript since version 3.7
In the example above, if a me.onChange property exists and is a function, it is called.
If no me.onChange property exists, nothing happens: the expression just returns undefined.
Note - if a me.onChange property exists but is not a function, a TypeError will be thrown just like when you call any non-function as a function in JavaScript. Optional Chaining doesn't do any magic to make this go away.

How about:
if('functionName' in Obj){
//code
}
e.g.
var color1 = new String("green");
"length" in color1 // returns true
"indexOf" in color1 // returns true
"blablabla" in color1 // returns false
or as for your case:
if('onChange' in me){
//code
}
See MDN docs.

If you're using eval to convert a string to function, and you want to check if this eval'd method exists, you'll want to use typeof and your function string inside an eval:
var functionString = "nonexsitantFunction"
eval("typeof " + functionString) // returns "undefined" or "function"
Don't reverse this and try a typeof on eval. If you do a ReferenceError will be thrown:
var functionString = "nonexsitantFunction"
typeof(eval(functionString)) // returns ReferenceError: [function] is not defined

Try typeof -- Look for 'undefined' to say it doesn't exist, 'function' for a function. JSFiddle for this code
function thisishere() {
return false;
}
alert("thisishere() is a " + typeof thisishere);
alert("thisisnthere() is " + typeof thisisnthere);
Or as an if:
if (typeof thisishere === 'function') {
// function exists
}
Or with a return value, on a single line:
var exists = (typeof thisishere === 'function') ? "Value if true" : "Value if false";
var exists = (typeof thisishere === 'function') // Returns true or false

Didn't see this suggested:
me.onChange && me.onChange(str);
Basically if me.onChange is undefined (which it will be if it hasn't been initiated) then it won't execute the latter part. If me.onChange is a function, it will execute me.onChange(str).
You can even go further and do:
me && me.onChange && me.onChange(str);
in case me is async as well.

For me the easiest way :
function func_exists(fname)
{
return (typeof window[fname] === 'function');
}

Put double exclamation mark i.e !! before the function name that you want to check. If it exists, it will return true.
function abc(){
}
!!window.abc; // return true
!!window.abcd; // return false

//Simple function that will tell if the function is defined or not
function is_function(func) {
return typeof window[func] !== 'undefined' && $.isFunction(window[func]);
}
//usage
if (is_function("myFunction") {
alert("myFunction defined");
} else {
alert("myFunction not defined");
}

function function_exists(function_name)
{
return eval('typeof ' + function_name) === 'function';
}
alert(function_exists('test'));
alert(function_exists('function_exists'));
OR
function function_exists(func_name) {
// discuss at: http://phpjs.org/functions/function_exists/
// original by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
// improved by: Steve Clay
// improved by: Legaev Andrey
// improved by: Brett Zamir (http://brett-zamir.me)
// example 1: function_exists('isFinite');
// returns 1: true
if (typeof func_name === 'string') {
func_name = this.window[func_name];
}
return typeof func_name === 'function';
}

function js_to_as( str ){
if (me && me.onChange)
me.onChange(str);
}

I'll go 1 step further to make sure the property is indeed a function
function js_to_as( str ){
if (me && me.onChange && typeof me.onChange === 'function') {
me.onChange(str);
}
}

I like using this method:
function isFunction(functionToCheck) {
var getType = {};
return functionToCheck && getType.toString.call(functionToCheck) === '[object Function]';
}
Usage:
if ( isFunction(me.onChange) ) {
me.onChange(str); // call the function with params
}

I had the case where the name of the function varied according to a variable (var 'x' in this case) added to the functions name. This works:
if ( typeof window['afunction_'+x] === 'function' ) { window['afunction_'+x](); }

The Underscore.js library defines it in the isFunction method as this (which comments suggest may cater for some browser bugs)
typeof obj == 'function' || false
http://underscorejs.org/docs/underscore.html#section-143

If you're checking for a function that is a jQuery plugin, you need to use $.fn.myfunction
if (typeof $.fn.mask === 'function') {
$('.zip').mask('00000');
}

Here is a working and simple solution for checking existence of a function and triggering that function dynamically by another function;
Trigger function
function runDynamicFunction(functionname){
if (typeof window[functionname] == "function") { //check availability
window[functionname]("this is from the function it"); // run function and pass a parameter to it
}
}
and you can now generate the function dynamically maybe using php like this
function runThis_func(my_Parameter){
alert(my_Parameter +" triggerd");
}
now you can call the function using dynamically generated event
<?php
$name_frm_somware ="runThis_func";
echo "<input type='button' value='Button' onclick='runDynamicFunction(\"".$name_frm_somware."\");'>";
?>
the exact HTML code you need is
<input type="button" value="Button" onclick="runDynamicFunction('runThis_func');">

In a few words: catch the exception.
I am really surprised nobody answered or commented about Exception Catch on this post yet.
Detail: Here goes an example where I try to match a function which is prefixed by mask_ and suffixed by the form field "name". When JavaScript does not find the function, it should throw an ReferenceError which you can handle as you wish on the catch section.
function inputMask(input) {
try {
let maskedInput = eval("mask_"+input.name);
if(typeof maskedInput === "undefined")
return input.value;
else
return eval("mask_"+input.name)(input);
} catch(e) {
if (e instanceof ReferenceError) {
return input.value;
}
}
}

With no conditions
me.onChange=function(){};
function getID( swfID ){
if(navigator.appName.indexOf("Microsoft") != -1){
me = window[swfID];
}else{
me = document[swfID];
}
}
function js_to_as( str ){
me.onChange(str);
}

I would suspect that me is not getting correctly assigned onload.
Moving the get_ID call into the onclick event should take care of it.
Obviously you can further trap as previously mentioned:
function js_to_as( str) {
var me = get_ID('jsExample');
if (me && me.onChange) {
me.onChange(str);
}
}

I always check like this:
if(!myFunction){return false;}
just place it before any code that uses this function

This simple jQuery code should do the trick:
if (jQuery.isFunction(functionName)) {
functionName();
}

I have tried the accepted answer; however:
console.log(typeof me.onChange);
returns 'undefined'.
I've noticed that the specification states an event called 'onchange' instead of 'onChange' (notice the camelCase).
Changing the original accepted answer to the following worked for me:
if (typeof me.onchange === "function") {
// safe to use the function
}

I have also been looking for an elegant solution to this problem. After much reflection, I found this approach best.
const func = me.onChange || (str => {});
func(str);

I would suggest using:
function hasMethod(subject, methodName) {
return subject != null && typeof subject[methodName] == "function";
}
The first check subject != null filters out nullish values (null and undefined) which don't have any properties. Without this check subject[methodName] could throw an error:
TypeError: (undefined|null) has no properties
Checking for only a truthy value isn't enough, since 0 and "" are both falsy but do have properties.
After validating that subject is not nullish you can safely access the property and check if it matches typeof subject[methodName] == "function".
Applying this to your code you can now do:
if (hasMethod(me, "onChange")) {
me.onChange(str);
}

function sum(nb1,nb2){
return nb1+nb2;
}
try{
if(sum() != undefined){/*test if the function is defined before call it*/
sum(3,5); /*once the function is exist you can call it */
}
}catch(e){
console.log("function not defined");/*the function is not defined or does not exists*/
}

And then there is this...
( document.exitPointerLock || Function )();

Try this one:
Window.function_exists=function(function_name,scope){
//Setting default scope of none is provided
If(typeof scope === 'undefined') scope=window;
//Checking if function name is defined
If (typeof function_name === 'undefined') throw new
Error('You have to provide an valid function name!');
//The type container
var fn= (typeof scope[function_name]);
//Function type
If(fn === 'function') return true;
//Function object type
if(fn.indexOf('function')!== false) return true;
return false;
}
Be aware that I've write this with my cellphone
Might contain some uppercase issues and/or other corrections needed like for example functions name
If you want a function like PHP to check if the var is set:
Window.isset=function (variable_con){
If(typeof variable_con !== 'undefined') return true;
return false;
}

To illustrate the preceding answers, here a quick JSFiddle snippet :
function test () {
console.log()
}
console.log(typeof test) // >> "function"
// implicit test, in javascript if an entity exist it returns implcitly true unless the element value is false as :
// var test = false
if(test){ console.log(true)}
else{console.log(false)}
// test by the typeof method
if( typeof test === "function"){ console.log(true)}
else{console.log(false)}
// confirm that the test is effective :
// - entity with false value
var test2 = false
if(test2){ console.log(true)}
else{console.log(false)}
// confirm that the test is effective :
// - typeof entity
if( typeof test ==="foo"){ console.log(true)}
else{console.log(false)}
/* Expected :
function
true
true
false
false
*/

How do you check if a JavaScript Object is a DOM Object?

I'm trying to get:
document.createElement('div') //=> true
{tagName: 'foobar something'} //=> false
In my own scripts, I used to just use this since I never needed tagName as a property:
if (!object.tagName) throw ...;
So for the second object, I came up with the following as a quick solution -- which mostly works. ;)
The problem is, it depends on browsers enforcing read-only properties, which not all do.
function isDOM(obj) {
var tag = obj.tagName;
try {
obj.tagName = ''; // Read-only for DOM, should throw exception
obj.tagName = tag; // Restore for normal objects
return false;
} catch (e) {
return true;
}
}
Is there a good substitute?

This might be of interest:
function isElement(obj) {
try {
//Using W3 DOM2 (works for FF, Opera and Chrome)
return obj instanceof HTMLElement;
}
catch(e){
//Browsers not supporting W3 DOM2 don't have HTMLElement and
//an exception is thrown and we end up here. Testing some
//properties that all elements have (works on IE7)
return (typeof obj==="object") &&
(obj.nodeType===1) && (typeof obj.style === "object") &&
(typeof obj.ownerDocument ==="object");
}
}
It's part of the DOM, Level2.
Update 2: This is how I implemented it in my own library:
(the previous code didn't work in Chrome, because Node and HTMLElement are functions instead of the expected object. This code is tested in FF3, IE7, Chrome 1 and Opera 9).
//Returns true if it is a DOM node
function isNode(o){
return (
typeof Node === "object" ? o instanceof Node :
o && typeof o === "object" && typeof o.nodeType === "number" && typeof o.nodeName==="string"
);
}
//Returns true if it is a DOM element
function isElement(o){
return (
typeof HTMLElement === "object" ? o instanceof HTMLElement : //DOM2
o && typeof o === "object" && o !== null && o.nodeType === 1 && typeof o.nodeName==="string"
);
}

The accepted answer is a bit complicated, and does not detect all types of HTML elements. For example, SVG elements are not supported. In contrast, this answer works for HTML as well as SVG, etc.
See it in action here: https://jsfiddle.net/eLuhbu6r/
function isElement(element) {
return element instanceof Element || element instanceof HTMLDocument;
}
Cherry on top: the above code is IE8 compatible.

No need for hacks, you can just ask if an element is an instance of the DOM Element:
const isDOM = el => el instanceof Element

All solutions above and below (my solution including) suffer from possibility of being incorrect, especially on IE — it is quite possible to (re)define some objects/methods/properties to mimic a DOM node rendering the test invalid.
So usually I use the duck-typing-style testing: I test specifically for things I use. For example, if I want to clone a node I test it like this:
if(typeof node == "object" && "nodeType" in node &&
node.nodeType === 1 && node.cloneNode){
// most probably this is a DOM node, we can clone it safely
clonedNode = node.cloneNode(false);
}
Basically it is a little sanity check + the direct test for a method (or a property) I am planning to use.
Incidentally the test above is a good test for DOM nodes on all browsers. But if you want to be on the safe side always check the presence of methods and properties and verify their types.
EDIT: IE uses ActiveX objects to represent nodes, so their properties do not behave as true JavaScript object, for example:
console.log(typeof node.cloneNode); // object
console.log(node.cloneNode instanceof Function); // false
while it should return "function" and true respectively. The only way to test methods is to see if the are defined.

A simple way to test if a variable is a DOM element (verbose, but more traditional syntax :-)
function isDomEntity(entity) {
if(typeof entity === 'object' && entity.nodeType !== undefined){
return true;
}
else{
return false;
}
}
Or as HTMLGuy suggested (short and clean syntax):
const isDomEntity = entity =>
typeof entity === 'object' && entity.nodeType !== undefined

You could try appending it to a real DOM node...
function isDom(obj)
{
var elm = document.createElement('div');
try
{
elm.appendChild(obj);
}
catch (e)
{
return false;
}
return true;
}

How about Lo-Dash's _.isElement?
$ npm install lodash.iselement
And in the code:
var isElement = require("lodash.iselement");
isElement(document.body);

This is from the lovely JavaScript library MooTools:
if (obj.nodeName){
switch (obj.nodeType){
case 1: return 'element';
case 3: return (/\S/).test(obj.nodeValue) ? 'textnode' : 'whitespace';
}
}

The using the root detection found here, we can determine whether e.g. alert is a member of the object's root, which is then likely to be a window:
function isInAnyDOM(o) {
return (o !== null) && !!(o.ownerDocument && (o.ownerDocument.defaultView || o.ownerDocument.parentWindow).alert); // true|false
}
To determine whether the object is the current window is even simpler:
function isInCurrentDOM(o) {
return (o !== null) && !!o.ownerDocument && (window === (o.ownerDocument.defaultView || o.ownerDocument.parentWindow)); // true|false
}
This seems to be less expensive than the try/catch solution in the opening thread.
Don P

old thread, but here's an updated possibility for ie8 and ff3.5 users:
function isHTMLElement(o) {
return (o.constructor.toString().search(/\object HTML.+Element/) > -1);
}

var IsPlainObject = function ( obj ) { return obj instanceof Object && ! ( obj instanceof Function || obj.toString( ) !== '[object Object]' || obj.constructor.name !== 'Object' ); },
IsDOMObject = function ( obj ) { return obj instanceof EventTarget; },
IsDOMElement = function ( obj ) { return obj instanceof Node; },
IsListObject = function ( obj ) { return obj instanceof Array || obj instanceof NodeList; },
// In fact I am more likely t use these inline, but sometimes it is good to have these shortcuts for setup code

I think prototyping is not a very good solution but maybe this is the fastest one:
Define this code block;
Element.prototype.isDomElement = true;
HTMLElement.prototype.isDomElement = true;
than check your objects isDomElement property:
if(a.isDomElement){}
I hope this helps.

This could be helpful: isDOM
//-----------------------------------
// Determines if the #obj parameter is a DOM element
function isDOM (obj) {
// DOM, Level2
if ("HTMLElement" in window) {
return (obj && obj instanceof HTMLElement);
}
// Older browsers
return !!(obj && typeof obj === "object" && obj.nodeType === 1 && obj.nodeName);
}
In the code above, we use the double negation operator to get the boolean value of the object passed as argument, this way we ensure that each expression evaluated in the conditional statement be boolean, taking advantage of the Short-Circuit Evaluation, thus the function returns true or false

According to mdn
Element is the most general base class from which all objects in a Document inherit. It only has methods and properties common to all kinds of elements.
We can implement isElement by prototype. Here is my advice:
/**
* #description detect if obj is an element
* #param {*} obj
* #returns {Boolean}
* #example
* see below
*/
function isElement(obj) {
if (typeof obj !== 'object') {
return false
}
let prototypeStr, prototype
do {
prototype = Object.getPrototypeOf(obj)
// to work in iframe
prototypeStr = Object.prototype.toString.call(prototype)
// '[object Document]' is used to detect document
if (
prototypeStr === '[object Element]' ||
prototypeStr === '[object Document]'
) {
return true
}
obj = prototype
// null is the terminal of object
} while (prototype !== null)
return false
}
console.log(isElement(document)) // true
console.log(isElement(document.documentElement)) // true
console.log(isElement(document.body)) // true
console.log(isElement(document.getElementsByTagName('svg')[0])) // true or false, decided by whether there is svg element
console.log(isElement(document.getElementsByTagName('svg'))) // false
console.log(isElement(document.createDocumentFragment())) // false

I think that what you have to do is make a thorough check of some properties that will always be in a dom element, but their combination won't most likely be in another object, like so:
var isDom = function (inp) {
return inp && inp.tagName && inp.nodeName && inp.ownerDocument && inp.removeAttribute;
};

In Firefox, you can use the instanceof Node. That Node is defined in DOM1.
But that is not that easy in IE.
"instanceof ActiveXObject" only can tell that it is a native object.
"typeof document.body.appendChild=='object'" tell that it may be DOM object, but also can be something else have same function.
You can only ensure it is DOM element by using DOM function and catch if any exception. However, it may have side effect (e.g. change object internal state/performance/memory leak)

Perhaps this is an alternative? Tested in Opera 11, FireFox 6, Internet Explorer 8, Safari 5 and Google Chrome 16.
function isDOMNode(v) {
if ( v===null ) return false;
if ( typeof v!=='object' ) return false;
if ( !('nodeName' in v) ) return false;
var nn = v.nodeName;
try {
// DOM node property nodeName is readonly.
// Most browsers throws an error...
v.nodeName = 'is readonly?';
} catch (e) {
// ... indicating v is a DOM node ...
return true;
}
// ...but others silently ignore the attempt to set the nodeName.
if ( v.nodeName===nn ) return true;
// Property nodeName set (and reset) - v is not a DOM node.
v.nodeName = nn;
return false;
}
Function won't be fooled by e.g. this
isDOMNode( {'nodeName':'fake'} ); // returns false

You can see if the object or node in question returns a string type.
typeof (array).innerHTML === "string" => false
typeof (object).innerHTML === "string" => false
typeof (number).innerHTML === "string" => false
typeof (text).innerHTML === "string" => false
//any DOM element will test as true
typeof (HTML object).innerHTML === "string" => true
typeof (document.createElement('anything')).innerHTML === "string" => true

This is what I figured out:
var isHTMLElement = (function () {
if ("HTMLElement" in window) {
// Voilà. Quick and easy. And reliable.
return function (el) {return el instanceof HTMLElement;};
} else if ((document.createElement("a")).constructor) {
// We can access an element's constructor. So, this is not IE7
var ElementConstructors = {}, nodeName;
return function (el) {
return el && typeof el.nodeName === "string" &&
(el instanceof ((nodeName = el.nodeName.toLowerCase()) in ElementConstructors
? ElementConstructors[nodeName]
: (ElementConstructors[nodeName] = (document.createElement(nodeName)).constructor)))
}
} else {
// Not that reliable, but we don't seem to have another choice. Probably IE7
return function (el) {
return typeof el === "object" && el.nodeType === 1 && typeof el.nodeName === "string";
}
}
})();
To improve performance I created a self-invoking function that tests the browser's capabilities only once and assigns the appropriate function accordingly.
The first test should work in most modern browsers and was already discussed here. It just tests if the element is an instance of HTMLElement. Very straightforward.
The second one is the most interesting one. This is its core-functionality:
return el instanceof (document.createElement(el.nodeName)).constructor
It tests whether el is an instance of the construcor it pretends to be. To do that, we need access to an element's contructor. That's why we're testing this in the if-Statement. IE7 for example fails this, because (document.createElement("a")).constructor is undefined in IE7.
The problem with this approach is that document.createElement is really not the fastest function and could easily slow down your application if you're testing a lot of elements with it. To solve this, I decided to cache the constructors. The object ElementConstructors has nodeNames as keys with its corresponding constructors as values. If a constructor is already cached, it uses it from the cache, otherwise it creates the Element, caches its constructor for future access and then tests against it.
The third test is the unpleasant fallback. It tests whether el is an object, has a nodeType property set to 1 and a string as nodeName. This is not very reliable of course, yet the vast majority of users shouldn't even fall back so far.
This is the most reliable approach I came up with while still keeping performance as high as possible.

Test if obj inherits from Node.
if (obj instanceof Node){
// obj is a DOM Object
}
Node is a basic Interface from which HTMLElement and Text inherit.

For the ones using Angular:
angular.isElement
https://docs.angularjs.org/api/ng/function/angular.isElement

This will work for almost any browser. (No distinction between elements and nodes here)
function dom_element_check(element){
if (typeof element.nodeType !== 'undefined'){
return true;
}
return false;
}

differentiate a raw js object from a HTMLElement
function isDOM (x){
return /HTML/.test( {}.toString.call(x) );
}
use:
isDOM( {a:1} ) // false
isDOM( document.body ) // true
// OR
Object.defineProperty(Object.prototype, "is",
{
value: function (x) {
return {}.toString.call(this).indexOf(x) >= 0;
}
});
use:
o={}; o.is("HTML") // false
o=document.body; o.is("HTML") // true

here's a trick using jQuery
var obj = {};
var element = document.getElementById('myId'); // or simply $("#myId")
$(obj).html() == undefined // true
$(element).html() == undefined // false
so putting it in a function:
function isElement(obj){
return (typeOf obj === 'object' && !($(obj).html() == undefined));
}

Not to hammer on this or anything but for ES5-compliant browsers why not just:
function isDOM(e) {
return (/HTML(?:.*)Element/).test(Object.prototype.toString.call(e).slice(8, -1));
}
Won't work on TextNodes and not sure about Shadow DOM or DocumentFragments etc. but will work on almost all HTML tag elements.

A absolute right method, check target is a real html element
primary code:
(function (scope) {
if (!scope.window) {//May not run in window scope
return;
}
var HTMLElement = window.HTMLElement || window.Element|| function() {};
var tempDiv = document.createElement("div");
var isChildOf = function(target, parent) {
if (!target) {
return false;
}
if (parent == null) {
parent = document.body;
}
if (target === parent) {
return true;
}
var newParent = target.parentNode || target.parentElement;
if (!newParent) {
return false;
}
return isChildOf(newParent, parent);
}
/**
* The dom helper
*/
var Dom = {
/**
* Detect if target element is child element of parent
* #param {} target The target html node
* #param {} parent The the parent to check
* #returns {}
*/
IsChildOf: function (target, parent) {
return isChildOf(target, parent);
},
/**
* Detect target is html element
* #param {} target The target to check
* #returns {} True if target is html node
*/
IsHtmlElement: function (target) {
if (!X.Dom.IsHtmlNode(target)) {
return false;
}
return target.nodeType === 1;
},
/**
* Detect target is html node
* #param {} target The target to check
* #returns {} True if target is html node
*/
IsHtmlNode:function(target) {
if (target instanceof HTMLElement) {
return true;
}
if (target != null) {
if (isChildOf(target, document.documentElement)) {
return true;
}
try {
tempDiv.appendChild(target.cloneNode(false));
if (tempDiv.childNodes.length > 0) {
tempDiv.innerHTML = "";
return true;
}
} catch (e) {
}
}
return false;
}
};
X.Dom = Dom;
})(this);

Each DOMElement.constructor returns function HTML...Element() or [Object HTML...Element] so...
function isDOM(getElem){
if(getElem===null||typeof getElem==="undefined") return false;
var c = getElem.constructor.toString();
var html = c.search("HTML")!==-1;
var element = c.search("Element")!==-1;
return html&&element;
}

I have a special way to do this that has not yet been mentioned in the answers.
My solution is based on four tests. If the object passes all four, then it is an element:
The object is not null.
The object has a method called "appendChild".
The method "appendChild" was inherited from the Node class, and isn't just an imposter method (a user-created property with an identical name).
The object is of Node Type 1 (Element). Objects that inherit methods from the Node class are always Nodes, but not necessarily Elements.
Q: How do I check if a given property is inherited and isn't just an imposter?
A: A simple test to see if a method was truly inherited from Node is to first verify that the property has a type of "object" or "function". Next, convert the property to a string and check if the result contains the text "[Native Code]". If the result looks something like this:
function appendChild(){
[Native Code]
}
Then the method has been inherited from the Node object. See https://davidwalsh.name/detect-native-function
And finally, bringing all the tests together, the solution is:
function ObjectIsElement(obj) {
var IsElem = true;
if (obj == null) {
IsElem = false;
} else if (typeof(obj.appendChild) != "object" && typeof(obj.appendChild) != "function") {
//IE8 and below returns "object" when getting the type of a function, IE9+ returns "function"
IsElem = false;
} else if ((obj.appendChild + '').replace(/[\r\n\t\b\f\v\xC2\xA0\x00-\x1F\x7F-\x9F ]/ig, '').search(/\{\[NativeCode]}$/i) == -1) {
IsElem = false;
} else if (obj.nodeType != 1) {
IsElem = false;
}
return IsElem;
}

(element instanceof $ && element.get(0) instanceof Element) || element instanceof Element
This will check for even if it is a jQuery or JavaScript DOM element

The only way to guarentee you're checking an actual HTMLEement, and not just an object with the same properties as an HTML Element, is to determine if it inherits from Node, since its impossible to make a new Node() in JavaScript. (unless the native Node function is overwritten, but then you're out of luck). So:
function isHTML(obj) {
return obj instanceof Node;
}
console.log(
isHTML(test),
isHTML(ok),
isHTML(p),
isHTML(o),
isHTML({
constructor: {
name: "HTML"
}
}),
isHTML({
__proto__: {
__proto__: {
__proto__: {
__proto__: {
constructor: {
constructor: {
name: "Function"
},
name: "Node"
}
}
}
}
}
}),
)
<div id=test></div>
<blockquote id="ok"></blockquote>
<p id=p></p>
<br id=o>
<!--think of anything else you want--!>