getArryaKeyByPercent() .js

// http://jsfiddle.net/antonfisher/qky16fp3/

// For human:

/**
 * Get Array key by percentage
 *
 *  getArrayKeyByPercent(["a", "b", "c"], 0) ==> "a"
 *  getArrayKeyByPercent(["a", "b", "c"], 50) ==> "b"
 *  getArrayKeyByPercent(["a", "b", "c"], 90) ==> "c"
 *
 * @param {Array}  array
 * @param {Number} percent
 * @returns {String} key off array
 */
var getArrayKeyByPercent = function (array, percent) {
    array = array || [];
    percent = Math.max(0, Math.min(percent, 99));

    var index = Math.floor(array.length * percent / 100);

    return array[index];
};

// One-liner for machine:
var getArrayKeyByPercent = function (a, p) {
    return (a || [])[Math.floor((a || []).length * Math.max(0, Math.min(p, 99)) / 100)];
};

// Test

console.log("== TEST =======================================");
console.log("== LENGHT 0 ==");
console.log("-- undefined ==", getArrayKeyByPercent(0, 0));

console.log("== LENGHT 1 ==");
console.log("-- red ==", getArrayKeyByPercent(["red"], -1));
console.log("-- red ==", getArrayKeyByPercent(["red"], 0));
console.log("-- red ==", getArrayKeyByPercent(["red"], 1));
console.log("-- red ==", getArrayKeyByPercent(["red"], 99));
console.log("-- red ==", getArrayKeyByPercent(["red"], 100));
console.log("-- red ==", getArrayKeyByPercent(["red"], 101));

console.log("== LENGHT 2 ==");
console.log("-- red ==", getArrayKeyByPercent(["red", "green"], -1));
console.log("-- red ==", getArrayKeyByPercent(["red", "green"], 0));
console.log("-- red ==", getArrayKeyByPercent(["red", "green"], 1));
console.log("-- red ==", getArrayKeyByPercent(["red", "green"], 49));
console.log("-- green ==", getArrayKeyByPercent(["red", "green"], 51));
console.log("-- green ==", getArrayKeyByPercent(["red", "green"], 99));
console.log("-- green ==", getArrayKeyByPercent(["red", "green"], 100));
console.log("-- green ==", getArrayKeyByPercent(["red", "green"], 101));

console.log("== LENGHT 3 ==");
console.log("-- red ==", getArrayKeyByPercent(["red", "green", "blue"], -1));
console.log("-- red ==", getArrayKeyByPercent(["red", "green", "blue"], 0));
console.log("-- red ==", getArrayKeyByPercent(["red", "green", "blue"], 1));
console.log("-- red ==", getArrayKeyByPercent(["red", "green", "blue"], 33));
console.log("-- green ==", getArrayKeyByPercent(["red", "green", "blue"], 34));
console.log("-- green ==", getArrayKeyByPercent(["red", "green", "blue"], 66));
console.log("-- blue ==", getArrayKeyByPercent(["red", "green", "blue"], 67));
console.log("-- blue ==", getArrayKeyByPercent(["red", "green", "blue"], 99));
console.log("-- blue ==", getArrayKeyByPercent(["red", "green", "blue"], 100));
console.log("-- blue ==", getArrayKeyByPercent(["red", "green", "blue"], 101));