Checking If a Key Exists in a JavaScript Object

Checking if a key exists in a JavaScript object?

Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined?

var obj = { key: undefined };
console.log(obj["key"] !== undefined); // false, but the key exists!

check if key exist in object

Standard function style here for your reference. You should use .hasOwnProperty

study = {
  "005": {     
        "Gen" : ["F"],
        "vr" : "cs"      
   },
}
let gen;
function check() {
  if (study["005"].hasOwnProperty("Gen")) {
    gen = study["005"]["Gen"]
  } else
    gen = "Unknown"
}
check()
console.log(gen)

And what you want

var gen = study["005"].hasOwnProperty("Gen") ? study["005"]["Gen"] : "Unknown" ;

Checking if a key exists in a JS object

Use the in operator:

testArray = 'key1' in obj;

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Sidenote: What you got there, is actually no jQuery object, but just a plain JavaScript Object.

How do I check if an object has a key in JavaScript?

Try the JavaScript in operator.

if ('key' in myObj)

And the inverse.

if (!('key' in myObj))

Be careful! The in operator matches all object keys, including those in the object's prototype chain.

Use myObj.hasOwnProperty('key') to check an object's own keys and will only return true if key is available on myObj directly:

myObj.hasOwnProperty('key')

Unless you have a specific reason to use the in operator, using myObj.hasOwnProperty('key') produces the result most code is looking for.

How to check if a key exists in an object in javascript

Use the hasOwnProperty call:

if (!obj.hasOwnProperty(key)) {

}

how to check if key is exist in data or not

Returns true when key exists.

const zipExists = (key) => key in zipCode;

More info: Checking if a key exists in a JavaScript object?

How to check if an object key is in another object of objects

• Convert the allMembers object into an array of key value pairs using Object.entries.

• Filter the items that have value true using Array.prototype.filter.

• Transform the array back to an object using Object.fromEntries.

const 
  allMembers = {'-Lz8YxHiwp8QZW3TqAFn': {first: 'foo',last: 'bar',uid: '-Lz8YxHiwp8QZW3TqAFn'},'-Lz8YxHqQXWoaGOFRLrO': {first: 'foo',last: 'bar',uid: '-Lz8YxHqQXWoaGOFRLrO'},'-Lz8YxHsMItaaTVNyQRE': {first: 'foo',last: 'bar',uid: '-Lz8YxHsMItaaTVNyQRE'},'-Lz8YxHwuVBMWl0Go6C5': {first: 'foo',last: 'bar',uid: '-Lz8YxHwuVBMWl0Go6C5'},'-Lz8YxHy0S-QkDaE1PkX': {first: 'foo',last: 'bar',uid: '-Lz8YxHy0S-QkDaE1PkX'}},
  attendanceData = {'-Lz8YxHiwp8QZW3TqAFn': true,'-Lz8YxHqQXWoaGOFRLrO': true,'-Lz8YxHsMItaaTVNyQRE': true,'-Lz8YxHwuVBMWl0Go6C5': false,'-Lz8YxHy0S-QkDaE1PkX': true,'-Lz8YxIFA1XGVmaNfNr3': false,'-Lz8YxIJVZnIIj7RgEzg': false},
  filteredMembers = Object.fromEntries(
    Object.entries(allMembers).filter(([k]) => attendanceData[k])
  );

console.log(filteredMembers);

Is !obj[key] a good practice to check the existence of an object's property in JavaScript?

You should use the in operator:

"key" in obj // true, regardless of the actual value

if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty:

obj.hasOwnProperty("key") // true

For performance comparison between the methods that are in, hasOwnProperty and key is undefined, check this reference

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