Iterating through a range in both directions

问题

There is a very common and easy task of looped iteration through some range in both directions:

var currentIndex = 0;
var range = ['a', 'b', 'c', 'd', 'e', 'f'];

function getNextItem(direction) {
    currentIndex += direction;
    if (currentIndex >= range.length) { currentIndex = 0; }
    if (currentIndex < 0) { currentIndex = range.length-1; }

    return range[currentIndex];
}        
// get next "right" item
console.log(getNextItem(1));

// get next "left" item
console.log(getNextItem(-1));

The code above works perfectly, but I spent about an hour trying to get rid of double if check.

Is there any way to solve if without if? Sort of one-liner maybe?

回答1:

To turn the two ifs into one unconditional statement, you can add the range.length to the currentIndex and then use modulo:

var currentIndex = 0;
var range = ['a','b','c','d','e','f'];

function getNextItem(direction) {
    currentIndex = (currentIndex + direction + range.length) % range.length;
    return range[currentIndex];
}

// get next "right" item
console.log(getNextItem(1));
console.log(getNextItem(1));

// get next "left" item
console.log(getNextItem(-1));
console.log(getNextItem(-1));
console.log(getNextItem(-1));

console.log(getNextItem(4));
console.log(getNextItem(1));
console.log(getNextItem(1));
console.log(getNextItem(1));

来源：https://stackoverflow.com/questions/58142680/iterating-through-a-range-in-both-directions