How to loop through an array of strings backwards x amount of times and start over when the end is reached?

Question

I am trying to loop through an array of months 11 times backwards. Basically I have a current month (September 2019) and I want to go back 11 months thus ending up at October 2018. I have looked at so many code snippets but none capture the whole 'starting over' portion. The point of it all is to push these months into a dropdown menu. Thus having the current month and the previous 11 months.

I have already tried this code snippet. It just goes forward. When I set it up to where it goes backwards there is this endless loop that kills my Chrome.

Here is what I have so far:

var monthNames = ["January", "February", "March", "April", "May", "June",
  "July", "August", "September", "October", "November", "December"
];
var d = new Date();
console.log(d.getMonth() + " is the index");

var index = monthNames[d.getMonth()];
console.log(index + ' is the month');
let monthIndex = monthNames.indexOf(index);
var n = 11

var result = [];

for (var i = monthIndex, len = monthNames.length; i <= monthIndex + n; i++) {
    result.push(monthNames[(i + len) % len]);
}


console.log(result.join(", "));

It is going forwards in months when I want it to start at September and go backwards and end up at October.

Any help is appreciated. Thank you!

Answer 1

You could take the actual month as offset and subtract the index and get the rest with the remainder for the value of the month names.

 var monthNames = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"], offset = new Date().getMonth(), length = 12, result = Array.from({ length }, (_, i) => monthNames[(12 + offset - i) % 12]); console.log(result.join(", ")); 

Answer 2

If you want a Python-like logic, you can try JavaScript Proxies :

 const monthNames = [ "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" ]; const months = new Proxy(monthNames, { get(target, prop) { if (!isNaN(prop)) { prop = parseInt(prop, 10); if (prop < 0) { prop += target.length; } } return target[prop]; } }) let d = new Date() let start = d.getMonth() - 1 let n = 11 let end = start - n, result = [] for(let c = start; c >= end; c--){ result.push(months[c]) } console.log(result.join(", ")) 

This way you will only need the starting month and subtract 11 from itself, and you can easily call that range from months . (eg start 5, end 5-11)

Answer 3

One approach that works is to subtract one from the index each time, checking when you're at 0 to loop back to the end, and stop the loop when you get to one more than the starting point. Note that this will bring the loop to a premature end because your ending condition is true at the start - so you need to start at one less than that, having prepopulated the array with the current month.

I've also removed a lot of the unnecessary repetitions in your original code. The below is, I freely admit, not particular elegant, but it works and should be easily understandable.

 var monthNames = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" ]; var startMonth = new Date().getMonth(); var result = [monthNames[startMonth]]; if (startMonth == 0) { startMonth = monthNames.length; } for (var i = startMonth - 1; i != startMonth; i = (i == 0) ? monthNames.length : (i - 1)) { result.push(monthNames[i]); } console.log(result.join(", ")); 

Answer 4

Keep it simple:

const monthNames = ["January", "February", "March", "April", "May", "June",
  "July", "August", "September", "October", "November", "December"
];


const octoberIndex = monthNames.findIndex(month => month === "October");
const monthsFromOctoberToSeptember = monthNames.slice(octoberIndex, monthNames.length)
.concat(monthNames.slice(0, octoberIndex));

const result = monthsFromOctoberToSeptember.reverse();

console.log(result.join(', '));

September, August, July, June, May, April, March, February, January, December, November, October

Answer 5

Not much different than what suggested above but bit simpler to understand

var monthNames = ["January", "February", "March", "April", "May", "June",
  "July", "August", "September", "October", "November", "December"
];
var currentDate = new Date();
var currentMonthIndex = currentDate.getMonth();
var result = [];
var monthLength = 12;

for (var i = currentMonthIndex; i < monthLength; i--) {
  result.push(monthNames[i]);

  if (result.length === monthLength) {
    break;
  }

  if (i === 0) {
    i = monthLength;
  }  
}


console.log(result.join(", "));