Is there a “not in” operator in JavaScript for checking object properties?

Question

Is there any sort of "not in" operator in JavaScript to check if a property does not exist in an object? I couldn’t find anything about this around Google or Stack Overflow. Here’s a small snippet of code I’m working on where I need this kind of functionality:

var tutorTimes = {};

$(checked).each(function(idx){
  id = $(this).attr('class');

  if(id in tutorTimes){}
  else{
    //Rest of my logic will go here
  }
});

As you can see, I’d be putting everything into the else statement. It seems wrong to me to set up an if–else statement just to use the else portion.

Yuletide

I think you might want var id = ... in your function. – Friar

Answer 1

It seems wrong to me to set up an if/else statement just to use the else portion...

Just negate your condition, and you'll get the else logic inside the if:

if (!(id in tutorTimes)) { ... }

Ronnie

This style also fixes the JSHint "Confusing use of '!'" warning you'd get if you did if ( ! somekey in someobj ) – Mishear

Please note that in searches for the property name anywhere in the prototype chain. See my answer for more details. – Prevocalic

I understand this is currently the best solution, but does anyone else agree that this is kinda ugly? – Scaliger

If it's ugly then just wrap it in a function and give it a beautiful name 🙃 let keyExists = (key, obj) => key in obj – Truong

I totally agree. Indeed my naming skills could be improved too 😁. let hasProperty "looks" better – Truong

@mikemaccana, I'm pretty sure that's because (! somekey in someobj) becomes (false in someobj). – Oxidation

Answer 2

Personally I find

if (id in tutorTimes === false) { ... }

easier to read than

if (!(id in tutorTimes)) { ... }

but both will work.

Coraciiform

Thank you. I prefer the more direct version too. The other is fine but more chars to read and consider. "if something is false" is easy to understand – Aceydeucy

Answer 3

As already said by Jordão, just negate it:

if (!(id in tutorTimes)) { ... }

Note: The above test if tutorTimes has a property with the name specified in id, anywhere in the prototype chain. For example "valueOf" in tutorTimes returns true because it is defined in Object.prototype.

If you want to test if a property doesn't exist in the current object, use hasOwnProperty:

if (!tutorTimes.hasOwnProperty(id)) { ... }

Or if you might have a key that is hasOwnPropery you can use this:

if (!Object.prototype.hasOwnProperty.call(tutorTimes,id)) { ... }

If your environment supports ECMA-292 from July 2022, you can use the convenient alternative to Object.prototype.hasOwnProperty Object.hasOwn:

if (!Object.hasOwn(tutorTimes,id)) { ... }

Prevocalic

Is it any safer to wrap the key in quotes and use if(!tutorTimes.hasOwnProperty('id')) ...? – Cacie

@MajidFouladpour id is an variable that could have any value, 'id' is a string with the two letters i and d, so hasOwnProperty(id) checks if the property specified in the variable id exists, and hasOwnProperty('id') checks if there is a property named id. – Prevocalic

Answer 4

Two quick possibilities:

if(!('foo' in myObj)) { ... }

or

if(myObj['foo'] === undefined) { ... }

Apeldoorn

Use 'undefined' === typeof xxx instead. undefined is not a reserved word and is actually a global variable that can be overwritten (leading to hard to find bugs) – Finical

@hippietrail doesn't work...the parens are required after the "!" and around the 'foo' in myObj) – Hutment

myObj['foo'] could exist as a property and simply be set to undefined (i.e., with the statement myObj.foo = undefined). If you really want to see if the property itself doesn't exist, you need the !('foo' in myObj) notation. – Nolannolana

For the performance conscious among us, checking for myObj.foo === undefined is much faster (albeit potentially dangerous): jsperf.com/not-in-vs-is-undefined. – Till

@Finical I don't think that is a very good argument for not using === undefined. Lots of things can break if people abuse a programming language by doing things like, for example, overwriting undefined in JavaScript. Also, see: #8784010 – Lines

In 2011 @Finical was right that undefined was a variable that could be overwritten with another value. That was changed in ECMA-262 5.1 Edition from June 2011. – Prevocalic

You can overwrite a lot of stuff in Javascript. If someone redefines undefined, just fire them.... :) – Oxidation

Answer 5

you can set the condition to be false

if ((id in tutorTimes === false)) { ... }

Somerville

You don't need the inner parenthesis – Aceydeucy

Answer 6

not very readable but a quick short hand could be:

if (id in tutorTimes ^ 1) { ... }

when doing operation with XOR, false and true got converted into 0 and 1 respectively. hence that reverse the result.

Diffusion