双向迭代范围 [英] Iterating through a range in both directions

问题描述

在两个方向的某个范围内进行循环迭代是一项非常常见且简单的任务:

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));

上面的代码运行完美，但是我花了大约一个小时试图摆脱两次if检查.

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

有没有办法解决如果没有的话?可能是单线的?

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

推荐答案

要将两个if变成一个无条件语句，可以将range.length添加到currentIndex，然后使用取模:

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));

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