JAVASCRIPT

javascript check if key exists in object

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

If you want to check if a key doesn't exist, remember to use parenthesis:
!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj   // ERROR!  Equivalent to "false in obj"

Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty:
obj.hasOwnProperty("key") // true

check if a key exists in an object javascript

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

how to check if a key exists in an object javascript

!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj   // ERROR!  Equivalent to "false in obj"

how to check if a key is present in a dictionary in js

var obj = { key: undefined };
obj["key"] !== undefined // false, but the key exists!

Checking if a key exists in a JavaScript object?

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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!

if element in dict javascript

var obj = { key: undefined };
console.log("key" in obj); // true, regardless of the actual value
 Run code snippet

js check if object key exists

var obj = { key: undefined };
obj["key"] !== undefined // false, but the key exists!

js dictionary contains key

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