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Which function is better to cover most cases to check if a data is Object or not between 2 function below

I'm usually using this condition to check if a value of data is Object or not:
// function A
export const isObject = item => {
return item && item.constructor === Object;
};
since it is easier to understand, but my friend usually using this function to check data with the same condition as above isObject(value) {}:
// function B
export function isObject(value) {
return typeof value === "object" && !Array.isArray(value) && value !== null;
}
we got confused when there is a conflict when we want to merge to latest branch, so here is the question, Between function A & B which one is better to cover most case regarding object data?, Can you explain it too so I have a better knowledge about it.

Assuming that you want to identify plain objects that inherit from Object.prototype, and aren't subclasses of something else:
The first approach could be fooled because the .constructor property doesn't necessarily refer to the constructor function used to create the function - an object could assign something else to that property.
const isObject = item => {
return item && item.constructor === Object;
};
const a = [];
console.log(a.constructor == Array);
a.constructor = Object;
console.log(a.constructor === Object);
console.log(isObject(a));
The second approach could be fooled because there are other sorts of things than arrays that inherit from Object.prototype that aren't plain objects nor arrays - like HTMLElements and many, many other things.
function isObject(value) {
return typeof value === "object" && !Array.isArray(value) && value !== null;
}
console.log(isObject(document.body));
console.log(isObject(new Proxy({}, {})));
So I prefer Object.getPrototypeOf, with which you can be quite confident that the value's immediate internal prototype is Object.prototype, and that it's nothing else special.
const isObject = item => Object.getPrototypeOf(item) === Object.prototype;
console.log(isObject([]));
console.log(isObject({}));
console.log(isObject(document));
It could fail if Object.getPrototypeOf is overwritten, of course - but if you're at that point, pretty much nothing is trustworthy anyway.
Note that many things inherit from Object.prototype which will give true for this, such as Number.prototype - if you want to exclude those kinds, you could check if a .constructor property exists.

How to return an Object Value if you have a specific key in your Object? [duplicate]

How do I check if an object has a specific property in JavaScript?
Consider:
x = {'key': 1};
if ( x.hasOwnProperty('key') ) {
//Do this
}
Is that the best way to do it?

2022 UPDATE
Object.hasOwn()
Object.hasOwn() is recommended over Object.hasOwnProperty() because it works for objects created using Object.create(null) and with objects that have overridden the inherited hasOwnProperty() method. While it is possible to workaround these problems by calling Object.prototype.hasOwnProperty() on an external object, Object.hasOwn() is more intuitive.
Example
const object1 = {
prop: 'exists'
};
console.log(Object.hasOwn(object1, 'prop'));
// expected output: true
Original answer
I'm really confused by the answers that have been given - most of them are just outright incorrect. Of course you can have object properties that have undefined, null, or false values. So simply reducing the property check to typeof this[property] or, even worse, x.key will give you completely misleading results.
It depends on what you're looking for. If you want to know if an object physically contains a property (and it is not coming from somewhere up on the prototype chain) then object.hasOwnProperty is the way to go. All modern browsers support it. (It was missing in older versions of Safari - 2.0.1 and older - but those versions of the browser are rarely used any more.)
If what you're looking for is if an object has a property on it that is iterable (when you iterate over the properties of the object, it will appear) then doing: prop in object will give you your desired effect.
Since using hasOwnProperty is probably what you want, and considering that you may want a fallback method, I present to you the following solution:
var obj = {
a: undefined,
b: null,
c: false
};
// a, b, c all found
for ( var prop in obj ) {
document.writeln( "Object1: " + prop );
}
function Class(){
this.a = undefined;
this.b = null;
this.c = false;
}
Class.prototype = {
a: undefined,
b: true,
c: true,
d: true,
e: true
};
var obj2 = new Class();
// a, b, c, d, e found
for ( var prop in obj2 ) {
document.writeln( "Object2: " + prop );
}
function hasOwnProperty(obj, prop) {
var proto = obj.__proto__ || obj.constructor.prototype;
return (prop in obj) &&
(!(prop in proto) || proto[prop] !== obj[prop]);
}
if ( Object.prototype.hasOwnProperty ) {
var hasOwnProperty = function(obj, prop) {
return obj.hasOwnProperty(prop);
}
}
// a, b, c found in modern browsers
// b, c found in Safari 2.0.1 and older
for ( var prop in obj2 ) {
if ( hasOwnProperty(obj2, prop) ) {
document.writeln( "Object2 w/ hasOwn: " + prop );
}
}
The above is a working, cross-browser, solution to hasOwnProperty(), with one caveat: It is unable to distinguish between cases where an identical property is on the prototype and on the instance - it just assumes that it's coming from the prototype. You could shift it to be more lenient or strict, based upon your situation, but at the very least this should be more helpful.

With Underscore.js or (even better) Lodash:
_.has(x, 'key');
Which calls Object.prototype.hasOwnProperty, but (a) is shorter to type, and (b) uses "a safe reference to hasOwnProperty" (i.e. it works even if hasOwnProperty is overwritten).
In particular, Lodash defines _.has as:
function has(object, key) {
return object ? hasOwnProperty.call(object, key) : false;
}
// hasOwnProperty = Object.prototype.hasOwnProperty

You can use this (but read the warning below):
var x = {
'key': 1
};
if ('key' in x) {
console.log('has');
}
But be warned: 'constructor' in x will return true even if x is an empty object - same for 'toString' in x, and many others. It's better to use Object.hasOwn(x, 'key').

Note: the following is nowadays largely obsolete thanks to strict mode, and hasOwnProperty. The correct solution is to use strict mode and to check for the presence of a property using obj.hasOwnProperty. This answer predates both these things, at least as widely implemented (yes, it is that old). Take the following as a historical note.
Bear in mind that undefined is (unfortunately) not a reserved word in JavaScript if you’re not using strict mode. Therefore, someone (someone else, obviously) could have the grand idea of redefining it, breaking your code.
A more robust method is therefore the following:
if (typeof(x.attribute) !== 'undefined')
On the flip side, this method is much more verbose and also slower. :-/
A common alternative is to ensure that undefined is actually undefined, e.g. by putting the code into a function which accepts an additional parameter, called undefined, that isn’t passed a value. To ensure that it’s not passed a value, you could just call it yourself immediately, e.g.:
(function (undefined) {
… your code …
if (x.attribute !== undefined)
… mode code …
})();

if (x.key !== undefined)
Armin Ronacher seems to have already beat me to it, but:
Object.prototype.hasOwnProperty = function(property) {
return this[property] !== undefined;
};
x = {'key': 1};
if (x.hasOwnProperty('key')) {
alert('have key!');
}
if (!x.hasOwnProperty('bar')) {
alert('no bar!');
}
A safer, but slower solution, as pointed out by Konrad Rudolph and Armin Ronacher would be:
Object.prototype.hasOwnProperty = function(property) {
return typeof this[property] !== 'undefined';
};

Considering the following object in Javascript
const x = {key: 1};
You can use the in operator to check if the property exists on an object:
console.log("key" in x);
You can also loop through all the properties of the object using a for - in loop, and then check for the specific property:
for (const prop in x) {
if (prop === "key") {
//Do something
}
}
You must consider if this object property is enumerable or not, because non-enumerable properties will not show up in a for-in loop. Also, if the enumerable property is shadowing a non-enumerable property of the prototype, it will not show up in Internet Explorer 8 and earlier.
If you’d like a list of all instance properties, whether enumerable or not, you can use
Object.getOwnPropertyNames(x);
This will return an array of names of all properties that exist on an object.
Reflections provide methods that can be used to interact with Javascript objects. The static Reflect.has() method works like the in operator as a function.
console.log(Reflect.has(x, 'key'));
// expected output: true
console.log(Reflect.has(x, 'key2'));
// expected output: false
console.log(Reflect.has(object1, 'toString'));
// expected output: true
Finally, you can use the typeof operator to directly check the data type of the object property:
if (typeof x.key === "undefined") {
console.log("undefined");
}
If the property does not exist on the object, it will return the string undefined. Else it will return the appropriate property type. However, note that this is not always a valid way of checking if an object has a property or not, because you could have a property that is set to undefined, in which case, using typeof x.key would still return true (even though the key is still in the object).
Similarly, you can check if a property exists by comparing directly to the undefined Javascript property
if (x.key === undefined) {
console.log("undefined");
}
This should work unless key was specifically set to undefined on the x object

Let's cut through some confusion here. First, let's simplify by assuming hasOwnProperty already exists; this is true of the vast majority of current browsers in use.
hasOwnProperty returns true if the attribute name that is passed to it has been added to the object. It is entirely independent of the actual value assigned to it which may be exactly undefined.
Hence:
var o = {}
o.x = undefined
var a = o.hasOwnProperty('x') // a is true
var b = o.x === undefined // b is also true
However:
var o = {}
var a = o.hasOwnProperty('x') // a is now false
var b = o.x === undefined // b is still true
The problem is what happens when an object in the prototype chain has an attribute with the value of undefined? hasOwnProperty will be false for it, and so will !== undefined. Yet, for..in will still list it in the enumeration.
The bottom line is there is no cross-browser way (since Internet Explorer doesn't expose __prototype__) to determine that a specific identifier has not been attached to an object or anything in its prototype chain.

If you are searching for a property, then "no". You want:
if ('prop' in obj) { }
In general, you should not care whether or not the property comes from the prototype or the object.
However, because you used 'key' in your sample code, it looks like you are treating the object as a hash, in which case your answer would make sense. All of the hashes keys would be properties in the object, and you avoid the extra properties contributed by the prototype.
John Resig's answer was very comprehensive, but I thought it wasn't clear. Especially with when to use "'prop' in obj".

For testing simple objects, use:
if (obj[x] !== undefined)
If you don't know what object type it is, use:
if (obj.hasOwnProperty(x))
All other options are slower...
Details
A performance evaluation of 100,000,000 cycles under Node.js to the five options suggested by others here:
function hasKey1(k,o) { return (x in obj); }
function hasKey2(k,o) { return (obj[x]); }
function hasKey3(k,o) { return (obj[x] !== undefined); }
function hasKey4(k,o) { return (typeof(obj[x]) !== 'undefined'); }
function hasKey5(k,o) { return (obj.hasOwnProperty(x)); }
The evaluation tells us that unless we specifically want to check the object's prototype chain as well as the object itself, we should not use the common form:
if (X in Obj)...
It is between 2 to 6 times slower depending on the use case
hasKey1 execution time: 4.51 s
hasKey2 execution time: 0.90 s
hasKey3 execution time: 0.76 s
hasKey4 execution time: 0.93 s
hasKey5 execution time: 2.15 s
Bottom line, if your Obj is not necessarily a simple object and you wish to avoid checking the object's prototype chain and to ensure x is owned by Obj directly, use if (obj.hasOwnProperty(x))....
Otherwise, when using a simple object and not being worried about the object's prototype chain, using if (typeof(obj[x]) !== 'undefined')... is the safest and fastest way.
If you use a simple object as a hash table and never do anything kinky, I would use if (obj[x])... as I find it much more readable.

Yes it is :) I think you can also do Object.prototype.hasOwnProperty.call(x, 'key') which should also work if x has a property called hasOwnProperty :)
But that tests for own properties. If you want to check if it has an property that may also be inhered you can use typeof x.foo != 'undefined'.

if(x.hasOwnProperty("key")){
// …
}
because
if(x.key){
// …
}
fails if x.key is falsy (for example, x.key === "").

You can also use the ES6 Reflect object:
x = {'key': 1};
Reflect.has( x, 'key'); // returns true
Documentation on MDN for Reflect.has can be found here.
The static Reflect.has() method works like the in operator as a function.

Do not do this object.hasOwnProperty(key)). It's really bad because these methods may be shadowed by properties on the object in question - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)).
The best way is to do Object.prototype.hasOwnProperty.call(object, key) or:
const has = Object.prototype.hasOwnProperty; // Cache the lookup once, in module scope.
console.log(has.call(object, key));
/* Or */
import has from 'has'; // https://www.npmjs.com/package/has
console.log(has(object, key));

OK, it looks like I had the right answer unless if you don't want inherited properties:
if (x.hasOwnProperty('key'))
Here are some other options to include inherited properties:
if (x.key) // Quick and dirty, but it does the same thing as below.
if (x.key !== undefined)

Another relatively simple way is using Object.keys. This returns an array which means you get all of the features of an array.
var noInfo = {};
var info = {something: 'data'};
Object.keys(noInfo).length //returns 0 or false
Object.keys(info).length //returns 1 or true
Although we are in a world with great browser support. Because this question is so old I thought I'd add this:
This is safe to use as of JavaScript v1.8.5.

JavaScript is now evolving and growing as it now has good and even efficient ways to check it.
Here are some easy ways to check if object has a particular property:
Using hasOwnProperty()
const hero = {
name: 'Batman'
};
hero.hasOwnProperty('name'); // => true
hero.hasOwnProperty('realName'); // => false
Using keyword/operator in
const hero = {
name: 'Batman'
};
'name' in hero; // => true
'realName' in hero; // => false
Comparing with undefined keyword
const hero = {
name: 'Batman'
};
hero.name; // => 'Batman'
hero.realName; // => undefined
// So consider this
hero.realName == undefined // => true (which means property does not exists in object)
hero.name == undefined // => false (which means that property exists in object)
For more information, check here.

hasOwnProperty "can be used to determine whether an object has the specified property as a direct property of that object; unlike the in operator, this method does not check down the object's prototype chain."
So most probably, for what seems by your question, you don't want to use hasOwnProperty, which determines if the property exists as attached directly to the object itself,.
If you want to determine if the property exists in the prototype chain, you may want to use it like:
if (prop in object) { // Do something }

You can use the following approaches-
var obj = {a:1}
console.log('a' in obj) // 1
console.log(obj.hasOwnProperty('a')) // 2
console.log(Boolean(obj.a)) // 3
The difference between the following approaches are as follows-
In the first and third approach we are not just searching in object but its prototypal chain too. If the object does not have the property, but the property is present in its prototype chain it is going to give true.
var obj = {
a: 2,
__proto__ : {b: 2}
}
console.log('b' in obj)
console.log(Boolean(obj.b))
The second approach will check only for its own properties. Example -
var obj = {
a: 2,
__proto__ : {b: 2}
}
console.log(obj.hasOwnProperty('b'))
The difference between the first and the third is if there is a property which has value undefined the third approach is going to give false while first will give true.
var obj = {
b : undefined
}
console.log(Boolean(obj.b))
console.log('b' in obj);

Given myObject object and “myKey” as key name:
Object.keys(myObject).includes('myKey')
or
myObject.hasOwnProperty('myKey')
or
typeof myObject.myKey !== 'undefined'
The last was widely used, but (as pointed out in other answers and comments) it could also match on keys deriving from Object prototype.

Performance
Today 2020.12.17 I perform tests on MacOs HighSierra 10.13.6 on Chrome v87, Safari v13.1.2 and Firefox v83 for chosen solutions.
Results
I compare only solutions A-F because they give valid result for all cased used in snippet in details section. For all browsers
solution based on in (A) is fast or fastest
solution (E) is fastest for chrome for big objects and fastest for firefox for small arrays if key not exists
solution (F) is fastest (~ >10x than other solutions) for small arrays
solutions (D,E) are quite fast
solution based on losash has (B) is slowest
Details
I perform 4 tests cases:
when object has 10 fields and searched key exists - you can run it HERE
when object has 10 fields and searched key not exists - you can run it HERE
when object has 10000 fields and searched key exists - you can run it HERE
when object has 10000 fields and searched key exists - you can run it HERE
Below snippet presents differences between solutions
A
B
C
D
E
F
G
H
I
J
K
// SO https://stackoverflow.com/q/135448/860099
// src: https://stackoverflow.com/a/14664748/860099
function A(x) {
return 'key' in x
}
// src: https://stackoverflow.com/a/11315692/860099
function B(x) {
return _.has(x, 'key')
}
// src: https://stackoverflow.com/a/40266120/860099
function C(x) {
return Reflect.has( x, 'key')
}
// src: https://stackoverflow.com/q/135448/860099
function D(x) {
return x.hasOwnProperty('key')
}
// src: https://stackoverflow.com/a/11315692/860099
function E(x) {
return Object.prototype.hasOwnProperty.call(x, 'key')
}
// src: https://stackoverflow.com/a/136411/860099
function F(x) {
function hasOwnProperty(obj, prop) {
var proto = obj.__proto__ || obj.constructor.prototype;
return (prop in obj) &&
(!(prop in proto) || proto[prop] !== obj[prop]);
}
return hasOwnProperty(x,'key')
}
// src: https://stackoverflow.com/a/135568/860099
function G(x) {
return typeof(x.key) !== 'undefined'
}
// src: https://stackoverflow.com/a/22740939/860099
function H(x) {
return x.key !== undefined
}
// src: https://stackoverflow.com/a/38332171/860099
function I(x) {
return !!x.key
}
// src: https://stackoverflow.com/a/41184688/860099
function J(x) {
return !!x['key']
}
// src: https://stackoverflow.com/a/54196605/860099
function K(x) {
return Boolean(x.key)
}
// --------------------
// TEST
// --------------------
let x1 = {'key': 1};
let x2 = {'key': "1"};
let x3 = {'key': true};
let x4 = {'key': []};
let x5 = {'key': {}};
let x6 = {'key': ()=>{}};
let x7 = {'key': ''};
let x8 = {'key': 0};
let x9 = {'key': false};
let x10= {'key': undefined};
let x11= {'nokey': 1};
let b= x=> x ? 1:0;
console.log(' 1 2 3 4 5 6 7 8 9 10 11');
[A,B,C,D,E,F,G,H,I,J,K ].map(f=> {
console.log(
`${f.name} ${b(f(x1))} ${b(f(x2))} ${b(f(x3))} ${b(f(x4))} ${b(f(x5))} ${b(f(x6))} ${b(f(x7))} ${b(f(x8))} ${b(f(x9))} ${b(f(x10))} ${b(f(x11))} `
)})
console.log('\nLegend: Columns (cases)');
console.log('1. key = 1 ');
console.log('2. key = "1" ');
console.log('3. key = true ');
console.log('4. key = [] ');
console.log('5. key = {} ');
console.log('6. key = ()=>{} ');
console.log('7. key = "" ');
console.log('8. key = 0 ');
console.log('9. key = false ');
console.log('10. key = undefined ');
console.log('11. no-key ');
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.20/lodash.min.js" integrity="sha512-90vH1Z83AJY9DmlWa8WkjkV79yfS2n2Oxhsi2dZbIv0nC4E6m5AbH8Nh156kkM7JePmqD6tcZsfad1ueoaovww==" crossorigin="anonymous"> </script>
This shippet only presents functions used in performance tests - it not perform tests itself!
And here are example results for chrome

Now with ECMAScript22 we can use hasOwn instead of hasOwnProperty (Because this feature has pitfalls )
Object.hasOwn(obj, propKey)

Here is another option for a specific case. :)
If you want to test for a member on an object and want to know if it has been set to something other than:
''
false
null
undefined
0
...
then you can use:
var foo = {};
foo.bar = "Yes, this is a proper value!";
if (!!foo.bar) {
// member is set, do something
}

some easier and short options depending on the specific use case:
to check if the property exists, regardless of value, use the in operator ("a" in b)
to check a property value from a variable, use bracket notation (obj[v])
to check a property value as truthy, use optional
chaining (?.)
to check a property value boolean, use double-not / bang-bang / (!!)
to set a default value for null / undefined check, use nullish coalescing operator (??)
to set a default value for falsey value check, use short-circuit logical OR operator (||)
run the code snippet to see results:
let obj1 = {prop:undefined};
console.log(1,"prop" in obj1);
console.log(1,obj1?.prop);
let obj2 = undefined;
//console.log(2,"prop" in obj2); would throw because obj2 undefined
console.log(2,"prop" in (obj2 ?? {}))
console.log(2,obj2?.prop);
let obj3 = {prop:false};
console.log(3,"prop" in obj3);
console.log(3,!!obj3?.prop);
let obj4 = {prop:null};
let look = "prop"
console.log(4,"prop" in obj4);
console.log(4,obj4?.[look]);
let obj5 = {prop:true};
console.log(5,"prop" in obj5);
console.log(5,obj5?.prop === true);
let obj6 = {otherProp:true};
look = "otherProp"
console.log(6,"prop" in obj6);
console.log(6,obj6.look); //should have used bracket notation
let obj7 = {prop:""};
console.log(7,"prop" in obj7);
console.log(7,obj7?.prop || "empty");
I see very few instances where hasOwn is used properly, especially given its inheritance issues

There is a method, "hasOwnProperty", that exists on an object, but it's not recommended to call this method directly, because it might be sometimes that the object is null or some property exist on the object like: { hasOwnProperty: false }
So a better way would be:
// Good
var obj = {"bar": "here bar desc"}
console.log(Object.prototype.hasOwnProperty.call(obj, "bar"));
// Best
const has = Object.prototype.hasOwnProperty; // Cache the lookup once, in module scope.
console.log(has.call(obj, "bar"));

An ECMAScript 6 solution with reflection. Create a wrapper like:
/**
Gets an argument from array or object.
The possible outcome:
- If the key exists the value is returned.
- If no key exists the default value is returned.
- If no default value is specified an empty string is returned.
#param obj The object or array to be searched.
#param key The name of the property or key.
#param defVal Optional default version of the command-line parameter [default ""]
#return The default value in case of an error else the found parameter.
*/
function getSafeReflectArg( obj, key, defVal) {
"use strict";
var retVal = (typeof defVal === 'undefined' ? "" : defVal);
if ( Reflect.has( obj, key) ) {
return Reflect.get( obj, key);
}
return retVal;
} // getSafeReflectArg

Showing how to use this answer
const object= {key1: 'data', key2: 'data2'};
Object.keys(object).includes('key1') //returns true
We can use indexOf as well, I prefer includes

You need to use the method object.hasOwnProperty(property). It returns true if the object has the property and false if the object doesn't.

The hasOwnProperty() method returns a boolean indicating whether the object has the specified property as its own property (as opposed to inheriting it).
const object1 = {};
object1.property1 = 42;
console.log(object1.hasOwnProperty('property1'));
// expected output: true
console.log(object1.hasOwnProperty('toString'));
// expected output: false
console.log(object1.hasOwnProperty('hasOwnProperty'));
// expected output: false
Know more

Don't over-complicate things when you can do:
var isProperty = (objectname.keyname || "") ? true : false;
It Is simple and clear for most cases...

A Better approach for iterating on object's own properties:
If you want to iterate on object's properties without using hasOwnProperty() check,
use for(let key of Object.keys(stud)){} method:
for(let key of Object.keys(stud)){
console.log(key); // will only log object's Own properties
}
full Example and comparison with for-in with hasOwnProperty()
function Student() {
this.name = "nitin";
}
Student.prototype = {
grade: 'A'
}
let stud = new Student();
// for-in approach
for(let key in stud){
if(stud.hasOwnProperty(key)){
console.log(key); // only outputs "name"
}
}
//Object.keys() approach
for(let key of Object.keys(stud)){
console.log(key);
}

typeof object but not array

I am looking for a quick check to determine if a value is an object {} but not an array []. I have written this:
function isPlainObject(input) {
return !Array.isArray(input) && typeof input === 'object';
}
Is there a shorter check I can use to determine this?
Or are there other possible data structures that still checkout as typeof 'object'?

It is not quicker, but more precise, with a check for falsy values, like null, which is an object.
function isPlainObject(input) {
return input && !Array.isArray(input) && typeof input === 'object';
}

If you want to check if an object is a "plain" object, i.e. inherits directly from Object.prototype, then you should check for that.
E.g. the following first tests if value has Object anywhere on it's prototype chain (and hence will not throw an error for getPrototypeOf), then checks if its immediate [[prototype]] is Object.prototype:
function isPlainObject(value) {
return value instanceof Object &&
Object.getPrototypeOf(value) == Object.prototype;
}
// Some tests
[[1,2], // Array
{}, // Plain object
null, // null
document.createElement('div'), // host object
function(){}, // function object
console // host objet
].forEach(function(value) {
console.log(value + ': ' + isPlainObject(value));
});
Edit
If you want to test that the input is some extended object but not a Function, etc. that is much less efficient, e.g. test against some list of objects that you want to avoid:
function isJustObj(obj) {
var notThese = [Function, Array, Date];
if (obj instanceof Object) {
return !notThese.some(function(o) {
return obj instanceof o;
});
}
return false;
}
function Foo(){}
var tests = {'Array: ':[],
'Object: ' : {},
'Foo instance:' : new Foo(),
'Function: ' : function(){},
'Date: ' : new Date(),
'Host obj: ' : document.createElement('div')
};
Object.keys(tests).forEach(function(test) {
console.log(test + isJustObj(tests[test]));
})
Note that this strategy sees if the value is some kind of Object, then tests whether it's an instance of a particular set of constructors. This list of things to exclude can become very large since it's not possible in any reasonable way to rule out host objects which, by their very nature, can be indistinguishable from built-in objects based on some general test (see Is there an environment-agnostic way to detect Javascript Host Objects?).
E.g.
console.log instanceof Function // true
console instanceof Object // true
isPlainObject(console) // false
So you either check if Object.prototype is the immediate [[Prototype]] or create a long list of constructors to test against. That list will go out of date very quickly given the variety of host environments available and the freedom for implementors to extend it. Also, you need to test every member of the host object set before trying to use it as it may not exist for the particular host on which the code is running.

Javascript arrays are considered objects so typeof will always be object in case of array.

A while loop and the "this" object

The code below defines a custom method for the Object's prototype that uses the native method "hasOwnProperty" to find the owner of the passed in property.
Object.prototype.findOwnerOfProperty = function(propName){
var currentObject = this;
while (currentObject !==null){
if (currentObject.hasOwnProperty(propName)){
return currentObject;
}
}
}
My encounters with while loops have been usually of this format:
while ( x < 10 ){
// do stuff
x++;
}
Say I called the "findOwnerOfProperty" method on an object:
newObject.findOwnerofProperty(firstProp);
My questions are:
1) What happens to the "this" object while the loop is running?
2) What exactly is the loop iterating through?
3) What is the difference between the first while loop and the second while loop, where the second loop has an obvious increment that explicitly changes the counter 'x' and the first loop doesnt? Which part of the first code changes the "currentObject"?

What is the difference between the first while loop and the second while loop
The first while loop is an infinite loop because currentObject never changes.

Property names are resolved firstly on the object itself, then on the objects on it's [[Prototype]] chain. You can access that chain using Object.getPrototypeOf, so you might be able to do something like:
Object.prototype.findOwnerOfProperty = function(propName) {
var obj = this;
do {
if (obj.hasOwnProperty(propName)) {
return obj;
}
obj = Object.getPrototypeOf(obj);
} while (obj)
}
// Some tests
var obj = {foo:'foo'};
var x = obj.findOwnerOfProperty('foo');
console.log(x == obj); // true
// Find foo on Bar.prototype
function Bar(){}
Bar.prototype.foo = 'foo';
var bar = new Bar();
var p = Object.getPrototypeOf(bar);
console.log(bar.findOwnerOfProperty('foo') == Bar.prototype); // true
// Find toString on Object.prototpye
console.log(bar.findOwnerOfProperty('toString') === Object.prototype); // true
// Non-existant property
console.log(bar.fum); // undefined
console.log(bar.findOwnerOfProperty('fum')); // undefined
The above returns undefined if no such object is found, which seems appropriate given that null is at the end of all [[Prototype]] chains and returning null would suggest that the property was found there.
Note that Object.getPrototypeOf is ES5 so not in all browsers in use.
Edit
It's possible that the function will be called with a value of this that isn't an Object, e.g.:
bar.findOwnerOfProperty.call(null, 'bar');
The desired outcome might be undefined or perhaps a type error, however the actual result depends on whether the code is strict or not and the value provided.
Non–strict code—if this is a primitive, then it will be set to the result of applying the abstract ToObject operator to the primitive value (e.g. if it's a number, then effectively new Number(value), if it's a string, then new String(value)).
In the case of null and undefined, this is set to the global object (note that applying ToObject to null or undefined throws an error) so the wrong inheritance chain will be checked (i.e. the global object, not null) and possibly the global object will be returned.
The fix for both these cases is "RTFM" (well, if there was one…) since by the time any code is executed, this has already been set and it's impossible to check the original call.
Strict code—in this case the value of this is not modified so a check can be made to ensure it's an Object or Function and return undefined otherwise:
Object.prototype.findOwnerOfProperty = function(propName) {
var obj = this;
// Only applies to strict mode
if ((typeof obj != 'object' && typeof obj != 'function') || obj === null) return;
while (obj) {
if (obj.hasOwnProperty(propName)) {
return obj;
}
obj = Object.getPrototypeOf(obj);
}
}
So there may be different results for strict and non–strict mode, e.g.
bar.findOwnerOfProperty.call(7, 'toString');
returns undefined for strict code and Number (i.e. the Number constructor) for non–strict code (because 7 is converted to a Number object as if by new Number(7), and calling typeof on a Number object returns 'object').
To achieve consistency, for values other than null and undefined, the ToObject operator could be emulated for strict code. Alternatively, the non–strict version could operate only on values where typeof returns function or object. I'll leave that decision to anyone who actually wants to implement this in anger.

if (key in object) or if(object.hasOwnProperty(key)

Do the following two statements produce the same output? Is there any reason to prefer one way to the other?
if (key in object)
if (object.hasOwnProperty(key))

Be careful - they won't produce the same result.
in will also return true if key gets found somewhere in the prototype chain, whereas Object.hasOwnProperty (like the name already tells us), will only return true if key is available on that object directly (its "owns" the property).

I'l try to explain with another example.
Say we have the following object with two properties:
function TestObj(){
this.name = 'Dragon';
}
TestObj.prototype.gender = 'male';
Let's create instance of TestObj:
var o = new TestObj();
Let's examine the object instance:
console.log(o.hasOwnProperty('name')); // true
console.log('name' in o); // true
console.log(o.hasOwnProperty('gender')); // false
console.log('gender' in o); // true
Conclusion:
in operator returns true always, if property is accessible by the object, directly or from the prototype
hasOwnProperty() returns true only if property exists on the instance, but not on its prototype
If we want to check that some property exist on the prototype, logically, we would say:
console.log(('name' in o) && !o.hasOwnProperty('name')); //false
console.log(('gender' in o) && !o.hasOwnProperty('gender')); //true - it's in prototype
Finally:
So, regarding to statement that these two conditions ...
if (key in object)
if (object.hasOwnProperty(key))
...produce the same result, the answer is obvious, it depends.

in will also check for inherited properties, which is not the case for hasOwnProperty.

In summary, hasOwnProperty() does not look in the prototype while in does look in the prototype.
Taken from O'Reilly High Performance Javascript:
You can determine whether an object has an instance member with a
given name by using the hasOwnProperty() method and passing in the
name of the member. To determine whether an object has access to a
property with a given name, you can use the in operator. For example:
var book = {
title: "High Performance JavaScript",
publisher: "Yahoo! Press"
};
alert(book.hasOwnProperty("title")); //true
alert(book.hasOwnProperty("toString")); //false
alert("title" in book); //true
alert("toString" in book); //true
In this code, hasOwnProperty() returns true when “title” is passed in
because title is an object instance; the method returns false when
“toString” is passed in because it doesn’t exist on the instance. When
each property name is used with the in operator, the result is true
both times because it searches the instance and prototype.

You got some really great answers.
I just want to offer something that will save you the need for checking "hasOwnProperty" while iterating an object.
When creating an object usually people will create it in this way:
const someMap = {}
// equivalent to: Object.create(Object.prototype)
// someMap.constructor will yield -> function Object() { [native code] }
Now, if you want to iterate through "someMap" you will have to do it this way:
const key
for(key in someMap ){
if (someMap.hasOwnProperty(key)) {
// Do something
}
}
We are doing so in order to avoid iterating over inherited properties.
If you intend to create a simple object that will only be used as a "map" (i.e. key - value pairs) you can do so like that:
const newMap = Object.create(null);
// Now, newMap won't have prototype at all.
// newMap.constructor will yield -> undefined
So now it will be safe to iterate like this:
for(key in cleanMap){
console.log(key + " -> " + newMap [key]);
// No need to add extra checks, as the object will always be clean
}
I learned this awesome tip here

The other form (called for in) enumerates the property names (or keys)
of an object. On each iteration, another property name string from the
object is assigned to the variable. It is usually necessary to test
object.hasOwnProperty(variable) to determine whether the property name
is truly a member of the object or was found instead on the prototype chain.
for (myvar in obj) {
if (obj.hasOwnProperty(myvar)) { ... } }
(from Crockford's Javascript: The Good Parts)

As other answers indicated, hasOwnProperty will check for an object own properties in contrast to in which will also check for inherited properties.
New method 2021 - Object.hasOwn() as a replacement for Object.hasOwnProperty()
Object.hasOwn() is intended as a replacement for Object.hasOwnProperty() and is a new method available to use (yet still not fully supported by all browsers like as you can see here - https://caniuse.com/?search=hasOwn
)
Object.hasOwn() is a static method which returns true if the specified object has the specified property as its own property. If the property is inherited, or does not exist, the method returns false.
const person = { name: 'dan' };
console.log(Object.hasOwn(person, 'name'));// true
console.log(Object.hasOwn(person, 'age'));// false
const person2 = Object.create({gender: 'male'});
console.log(Object.hasOwn(person2, 'gender'));// false
It is recommended to this method use over the Object.hasOwnProperty() because it also works for objects created by using Object.create(null) and for objects that have overridden the inherited hasOwnProperty() method. Although it's possible to solve these kind of problems by calling Object.prototype.hasOwnProperty() on an external object, Object.hasOwn() overcome these problems, hence is preferred (see examples below)
let person = {
hasOwnProperty: function() {
return false;
},
age: 35
};
if (Object.hasOwn(person, 'age')) {
console.log(person.age); // true - the remplementation of hasOwnProperty() did not affect the Object
}
let person = Object.create(null);
person.age = 35;
if (Object.hasOwn(person, 'age')) {
console.log(person.age); // true - works regardless of how the object was created
}
More about Object.hasOwn can be found here : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn
Browser compatibility - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn#browser_compatibility

Another way to have only ownproperties is :
<script type="text/javascript">"use strict";
const obj = Object.create({cle:"valeur"});
obj.a = "aaa";
obj.b = "bbb";
obj.c = "ccc";
for(let key=0 ; key < Object.keys(obj).length ; key++){
if(Object.keys(obj)[key]==="cle")
console.log(key , Object.keys(obj)[key] , Object.values(obj)[key]);
// none
if(Object.keys(obj)[key]==="b")
console.log(key , Object.keys(obj)[key] , Object.values(obj)[key]);
// 1 'b' 'bbb'
console.log(key , Object.keys(obj)[key] , Object.values(obj)[key]);
// 0 'a' 'aaa'
// 1 'b' 'bbb'
// 2 'c' 'ccc'
}
console.log(Object.getOwnPropertyDescriptor(obj,"cle"));
// undefined
console.log(Object.getOwnPropertyDescriptor(obj,"c"));
// {value:'ccc', writable:true, enumerable:true, configurable:true}
</script>

The first version is shorter (especially in minified code where the variables are renamed)
a in b
vs
b.hasOwnProperty(a)
Anyway, as #AndreMeinhold said, they do not always produce the same result.