@JLHwung If you follow the specs the precedence values is wrong :) The '??' should have lowest value.

There are a lot of docs and issue tickets about this, but here you can see it clearly

https://github.com/tc39/proposal-nullish-coalescing/blob/80a589d657c322fd2e152c1936bd9704904b2068/spec.html#L14-L17

That precedence statement does not follow from the grammar notation in the spec

ShortCircuitExpression[In, Yield, Await]:
  LogicalORExpression[?In, ?Yield, ?Await]
  CoalesceExpression[?In, ?Yield, ?Await]

That statement was updated in tc39/proposal-nullish-coalescing@99ff664 where a NullishExpression was introduced

NullishExpression[In, Yield, Await, Nullish] :
      LogicalORExpression[?In, ?Yield, ?Await, ?Nullish]
      NullishExpression[?In, ?Yield, ?Await, +Nullish] `??` LogicalORExpression[?In, ?Yield, ?Await, +Nullish]

apparently here ?? is lower than || in this notation. But later it was changed to ShortCircuitExpression, where

1. the RHS of CoalesceExpression is either CoalesceExpression or BitwiseORExpression.
2. the RHS of LogicalORExpression does not contain CoalesceExpression

So the grammar itself does not conclude that the precedence of coalesce is higher or lower than the logical_or. The only thing we know is that

both coalesce and logical_or are lower than bit_or. (f.1)
logical_or is lower than logical_and. (f.2)

Since the precedence of coalesce and logical_or is meant to solve the error of mixing coalesce and logical operators and now we throw when they are mixed, I don't think the exact precedence of coalesce and logical_or will have any impact as long as the precedence specification satisfies (f.1) and (f.2).

I can think of one advantage if you specify the precedence as coalesce: 1, or: 2, and: 3: you can now check if logical is mixed with coalesce by

(op.binop >> 1) ^ (nextOp.binop >> 1) === 1

This is similar to what is done in seafox but I think here code readability weighs more than performance, unless we do identify that parsing coalesce is the bottleneck.