JavaScript - "not in" operator for checking object properties?

You can just negate the condition that contains in operator:

if (!('propertyName' in objectName)) {
    // do something when the property is not in the object
}

Practical example

// ONLINE-RUNNER:browser;

const myObject = { a: 1, b: 2 };

if (!('c' in myObject)) {
    console.log('myObject does not contain the "c" property.');
}

References

1. in operator - JavaScript | MDN