Normal for loops can end early by using break or return. Might be useful to have the ability here too. Does that make sense?

Possible implementations:

1

Add a method for ending the iteration in the second argument to mapper: mapper(index, stop):

pTimes(5, (i, stop) => {
    if (shouldStop(i)) {
        stop();
        return;
    }

    return `foo ${i}`;
}).then();

2

Add ability to throw a special error to end the iteration without rejection the promise:

pTimes(5, i => {
    if (shouldStop(i)) {
        throw new pTimes.StopError();
    }

    return `foo ${i}`;
}).then();

3

Add ability to return special Symbol to end the iteration:

pTimes(5, i => {
    if (shouldStop(i)) {
        return pTimes.StopSymbol;
    }

    return `foo ${i}`;
}).then();

While 2 and 3 are more verbose, they have the benefit of working even in a nested promise chain.