(Read all before using)

The pending of 'build' for creating run exclusive functions that complete via calling a callback function. (Instead of returning a promise).

The only valid reason to use this instead of build() is to be able to retrieve the result and/or release the lock synchronously when it's possible.

If you want the callback to be optional it is possible to define the function as such:
const myRunExclusiveFunction = buildCb((callback?)=> { ... });
Anyway you must call it every time and assume it has been defined: callback!(...).

To see if the user has actually provided a callback you can access the hidden property callback.hasCallback.

WARNING: You must also make sure, if you use an optional callback that the argument before it cannot be a function.
Be aware that the compiler won't warn you against it.
Example: (getLetter:()=> string, callback?:(res:string)=> void)=>{..}
is NOT a valid function to pass to buildCb()

WARNING: the source function should NEVER throw exception!