RoundTo.js

let Size20 = [-30, -10, 10, 30, 50, 70, 90, 110, 130, 150];

let Size30 = [-40, -10, 20, 50, 80, 110, 140, 170, 200];

const assert = (target, source, title=`TEST ${target} === ${source}`) => {
    if(target === source){
        return {title, status: "SUCCESS"}
    }else{
      return {title, status: "FAILED"}
    }
}

roundToN = (number, offset= 0, size= 0) => {
    let multiplier = 1;
    let next = 0;
    let prev = 0;
    while (next < number) {
      multiplier += 1;
      next = offset + size * multiplier;
    }
    prev = number - size;

    console.log(`prev ${prev}, next${next}, size${size}, offset${offset}, number${number}`);
    const diff = number - prev;
    if (diff >= size / 2 ) {
      return next;
    }else {
      return prev;
    }
  }

  


assert(roundToN(57), 50);
assert(roundToN(72), 80);
assert(roundToN(86), 80);
assert(roundToN(105), 110);
assert(roundToN(-30), -40);
assert(roundToN(-22), -10);