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I am checking for the existence of an object property with a variable holding the property name in question.

var myObj;
myObj.prop = "exists";
var myProp = "p"+"r"+"o"+"p";

if(myObj.myProp){
    alert("yes, i have that property");
};

This is undefined because it's looking for myObj.myProp but I want it to check for myObj.prop

I am checking for the existence of an object property with a variable holding the property name in question.

var myObj;
myObj.prop = "exists";
var myProp = "p"+"r"+"o"+"p";

if(myObj.myProp){
    alert("yes, i have that property");
};

This is undefined because it's looking for myObj.myProp but I want it to check for myObj.prop

• javascript
• object
• typeguards

Share Improve this question Follow edited Jan 3, 2024 at 8:49 Penny Liu 17.3k55 gold badges8686 silver badges108108 bronze badges asked Jun 14, 2012 at 19:58 Slopeside CreativeSlopeside Creative 9,20744 gold badges1818 silver badges1818 bronze badges 4

• 3 Possibly useful: From a comment by Pablo Cabrera at NCZOnline: "I think it’s worth to note that if the hasOwnProperty method is overwritten, you can rely on the Object.prototype.hasOwnProperty.call(object, property)." – HumanInDisguise Commented Apr 7, 2015 at 8:45
• 12 is stackoverflow.com/questions/4244896/… a duplicate of this question? how is that? 'checking existence' and 'accessing value' are different things? Please correct me if I am wrong .... – adnan2nd Commented May 10, 2018 at 17:14
• this is not a duplicate. – Jeff Clayton Commented Sep 6, 2019 at 20:13
• @HumanInDisguise comments should not be used to provide resolving advice. Your comment would have been better placed as an answer which contains static quoted advice and a link to its source. Now that @ adnan2d has posted this advice, your comment can be safely deleted. – mickmackusa Commented Jun 9, 2021 at 2:05

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11 Answers 11

Sorted by: Reset to default 1587

var myProp = 'prop';
if(myObj.hasOwnProperty(myProp)){
    alert("yes, i have that property");
}

Or

var myProp = 'prop';
if(myProp in myObj){
    alert("yes, i have that property");
}

Or

if('prop' in myObj){
    alert("yes, i have that property");
}

Note that hasOwnProperty doesn't check for inherited properties, whereas in does. For example 'constructor' in myObj is true, but myObj.hasOwnProperty('constructor') is not.

You can use hasOwnProperty, but based on the reference you need quotes when using this method:

if (myObj.hasOwnProperty('myProp')) {
    // do something
}

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty

Another way is to use in operator, but you need quotes here as well:

if ('myProp' in myObj) {
    // do something
}

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in

Thank you for everyone's assistance and pushing to get rid of the eval statement. Variables needed to be in brackets, not dot notation. This works and is clean, proper code.

Each of these are variables: appChoice, underI, underObstr.

if(typeof tData.tonicdata[appChoice][underI][underObstr] !== "undefined"){
    //enter code here
}

For own property :

var loan = { amount: 150 };
if(Object.prototype.hasOwnProperty.call(loan, "amount")) 
{ 
   //will execute
}

Note: using Object.prototype.hasOwnProperty is better than loan.hasOwnProperty(..), in case a custom hasOwnProperty is defined in the prototype chain (which is not the case here), like

var foo = {
      hasOwnProperty: function() {
        return false;
      },
      bar: 'Here be dragons'
    };

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty

To include inherited properties in the finding use the in operator: (but you must place an object at the right side of 'in', primitive values will throw error, e.g. 'length' in 'home' will throw error, but 'length' in new String('home') won't)

const yoshi = { skulk: true };
const hattori = { sneak: true };
const kuma = { creep: true };
if ("skulk" in yoshi) 
    console.log("Yoshi can skulk");

if (!("sneak" in yoshi)) 
    console.log("Yoshi cannot sneak");

if (!("creep" in yoshi)) 
    console.log("Yoshi cannot creep");

Object.setPrototypeOf(yoshi, hattori);

if ("sneak" in yoshi)
    console.log("Yoshi can now sneak");
if (!("creep" in hattori))
    console.log("Hattori cannot creep");

Object.setPrototypeOf(hattori, kuma);

if ("creep" in hattori)
    console.log("Hattori can now creep");
if ("creep" in yoshi)
    console.log("Yoshi can also creep");

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in

Note: One may be tempted to use typeof and [ ] property accessor as the following code which doesn't work always ...

var loan = { amount: 150 };

loan.installment = undefined;

if("installment" in loan) // correct
{
    // will execute
}

if(typeof loan["installment"] !== "undefined") // incorrect
{
    // will not execute
}

A much more secure way to check if property exists on the object is to use empty object or object prototype to call hasOwnProperty()

var foo = {
  hasOwnProperty: function() {
    return false;
  },
  bar: 'Here be dragons'
};

foo.hasOwnProperty('bar'); // always returns false

// Use another Object's hasOwnProperty and call it with 'this' set to foo
({}).hasOwnProperty.call(foo, 'bar'); // true

// It's also possible to use the hasOwnProperty property from the Object
// prototype for this purpose
Object.prototype.hasOwnProperty.call(foo, 'bar'); // true

Reference from MDN Web Docs - Object.prototype.hasOwnProperty()

there are much simpler solutions and I don't see any answer to your actual question:

"it's looking for myObj.myProp but I want it to check for myObj.prop"

1. to obtain a property value from a variable, use bracket notation.
2. to test that property for truthy values, use optional chaining
3. to return a boolean, use double-not / bang-bang / (!!)
4. use the in operator if you are certain you have an object and only want to check for the existence of the property (true even if prop value is undefined). or perhaps combine with nullish coalescing operator ?? to avoid errors being thrown.

var nothing = undefined;
var obj = {prop:"hello world"}
var myProp = "prop";

consolelog( 1,()=> obj.myProp); // obj does not have a "myProp"
consolelog( 2,()=> obj[myProp]); // brackets works
consolelog( 3,()=> nothing[myProp]); // throws if not an object
consolelog( 4,()=> obj?.[myProp]); // optional chaining very nice ⭐️⭐️⭐️⭐️⭐️
consolelog( 5,()=> nothing?.[myProp]); // optional chaining avoids throwing
consolelog( 6,()=> nothing?.[nothing]); // even here it is error-safe
consolelog( 7,()=> !!obj?.[myProp]); // double-not yields true
consolelog( 8,()=> !!nothing?.[myProp]); // false because undefined
consolelog( 9,()=> myProp in obj); // in operator works
consolelog(10,()=> myProp in nothing); // throws if not an object
consolelog(11,()=> myProp in (nothing ?? {})); // safe from throwing
consolelog(12,()=> myProp in {prop:undefined}); // true (prop key exists even though its value undefined)

// beware of 'hasOwnProperty' pitfalls
// it can't detect inherited properties and 'hasOwnProperty' is itself inherited
// also comparatively verbose
consolelog(13,()=> obj.hasOwnProperty("hasOwnProperty")); // DANGER: it yields false 
consolelog(14,()=> nothing.hasOwnProperty("hasOwnProperty")); // throws when undefined
obj.hasOwnProperty = ()=>true; // someone could overwrite it
consolelog(15,()=> obj.hasOwnProperty(nothing)); // DANGER: the foolish overwrite will now always return true
consolelog(16,()=> Object.prototype.hasOwnProperty.call(obj,"hasOwnProperty")); //

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