What does ?. and ?? operator in javascript do? [duplicate]

Question

I recently noticed usage like this in a java script code base what does it do. I was unable to find any relevant documentation regarding that. Though intuitively operators seem to checking whether property is present. Is there any official documentation regarding this.

Ex:

args?.propertyName !== 'someValue'
const value = props.someProp ?? props.defaultProp;

Answer 1

They are for optionals:

val ?? other is called nullish coalescing operator and is equivalent to val == null ? other : val

and optionalThing?.property is refered as optional chaining and is the same as optionalThing == null ? null : optionalThing.property

This optional chaining expressions result in shorter and simpler expressions when accessing chained properties when the possibility exists that a reference may be missing ( allows you to do things like optionalThing?.optionalProperty?.anotherOptionalProperty?.property ).