Demonstratory JavaScript module for testing for the presence of elements within given ranges in a numeric array.

checkRanges.js

/*
 * The requirement for this module is to accept a number array
 * and test for the presence of at least one element in each of
 * the following ranges:
 * 
 *      10-20
 *      20-30
 *      30-40
 *      40-50
 *      50-60
 *      60-70
 *      70-80
 *      80-90
 * 
 * Each range includes the first number but not the second.
 * 
 * The first step is to start with an array of falses.  Each index
 * is this array corresponds to one of the ranges above.  False means
 * that no example has been found. True means that at least one has.
 * 
 * The initial value is all false since no items have been processed yet.
 */
const rangeArr = [false,false,false,false,false,false,false,false];


/* 
 * Now we need a helper function to process one number at a time.  This
 * function accepts an array of booleans and number.  It then tests if
 * that value is within one of the ranges listed above, and updates the
 * appropriate index in the boolean array.
 *
 * Note that this function copies the original input array to avoid mutating
 * it.  This will be very inefficient for larger arrays.
 */
function checkItem (arr, x) {
    
    const arr1 = arr.slice();
    let xIndex;
    
    if (x < 10) xIndex = -1;
    else if (x < 20) xIndex = 0;
    else if (x < 30) xIndex = 1;
    else if (x < 40) xIndex = 2;
    else if (x < 50) xIndex = 3;
    else if (x < 60) xIndex = 4;
    else if (x < 70) xIndex = 5;
    else if (x < 80) xIndex = 6;
    else if (x < 90) xIndex = 7;
    else xIndex = -1;
    
    if ((xIndex !== -1) && (!arr[xIndex]))
        arr1[xIndex] = true;
    
    return arr1;
    
}


/* 
 * This function updates the input array given two conditions:
 * 
 *      The given number is within the range 10 - 90 and,
 *      The index for that range is false.
 * 
 * It then returns the new, updated array.
 * 
 * To use this function, we can now use it as the first parameter to the
 * Array.prototype.reduce function.  This function applies the supplied
 * function to each item of the array in turn, and passes the return
 * value through to the next iteration.  We also provide the array of
 * falses to the function as the initial value.
 */

function checkRanges (arr) {
    
    return arr.reduce (checkItem, rangeArr);
    
}


/* 
 * The return value from this function will be an array of eight
 * booleans which will communicate whether an example of each range
 * has been encountered.  To find out if all items in this array are true,
 * we can use another helper function and reduce that.
 */
function bothTrue (x, y) {
    
    return x && y;
    
}


/* 
 * This function returns false if either parameter is false.  By reducing
 * over this, we can determine if any one of the elements in the boolean
 * array is false.
 */

function isAllTrue (arr) {
    
    return arr.reduce (bothTrue);
    
}


/* 
 * Note that we don't necessarily need to use that function, if the
 * intermediate result is not required.  Instead, we can simply reduce
 * over both helper functions to return a single boolean.
 * 
 * Putting it all together, the next function first reduces over 
 * the number array to get an array of booleans, then reduces over that to
 * get a single boolean value.
 */
function checkAll (arr) {
    
    return arr.reduce (checkItem, rangeArr)
            .reduce (bothTrue);
    
}


/*
 * Now we just need to export the functions.  For the purposes of this
 * demo we'll export everything, but this isn't necessarily how you would
 * want to it in production code.
 *
 * Of course, this is making the assumption that you're using Node.  If not,
 * you'll need to adapt this to your own use case.
 * 
 * Please note that this algorithm is not the fastest or most efficient.
 * One particular ineffifiency is that if the boolean array all true, the
 * reduce function will still run through to the end of the array, but it
 * should be logical enough to understand.  You will still need to do quite
 * a bit more work to make this into production-ready code.
 */
module.exports = {
    
    rangeArr,
    checkItem,
    checkRanges,
    bothTrue,
    isAllTrue,
    checkAll
    
}