?? is the Nullish coalescing operator.

a ?? b evaluates to:

• a if a is neither null nor undefined
• b otherwise

When chaining ?? such as a ?? b ?? c, you will get the first value from the left that is neither null nor undefined.

So in the example

const dealType = currentDealType ?? originalDealType ?? '';

dealType will be assigned to:

• currentDealType if currentDealType is neither null nor undefined
• otherwise originalDealType if originalDealType is neither null nor undefined
• otherwise ''

Note that a ?? b is different from a || b since

• ?? only checks for undefined and null
• || checks for any falsy value which includes undefined, null, 0, NaN and '' among others.