Get matched range item from array of objects javascript

I have one array which is named as tiers and one quantity variable. Now I am trying to get the matched range value from my tier array. Based on the matched result I want to calculate the discount. I tried to with find method but it gives me an unexpected ouput.

Actual output {id: 1, discount_percent:0, quantity:6, title:"Tier 1"}

Expeceted Output {id: 3, discount_percent:10, quantity:10, title:"Tier 3"}

let tiers = [
   {id: 1, discount_percent:0, quantity:6, title:"Tier 1"},
   {id: 2, discount_percent:5, quantity:8, title:"Tier 2"},
   {id: 3, discount_percent:10, quantity:10, title:"Tier 3"},
   {id: 4, discount_percent:12, quantity:12, title:"Tier 4"},
   {id: 5, discount_percent:14, quantity:14, title:"Tier 5"},
   {id: 6, discount_percent:20, quantity:16, title:"Tier 6"},
   {id: 7, discount_percent:40, quantity:18, title:"Tier 7"},
   {id: 8, discount_percent:50, quantity:50, title:"Tier 8"},
]

function calculateDiscount(){
   const ordersQuanity = 10;
   const tier = tiers.find((_tier) => _tier.quantity <= ordersQuanity);
   ...
}

Answer

For the generic situation of a number of objects with discount_percents and quantitys, .find isn’t the right approach because it’ll stop as soon as it finds a match. Consider .reduce instead – if an element being iterated over passes the test and it has a greater discount_percent than the current element in the accumulator (if there’s anything in the accumulator to begin with), return it instead.

let tiers = [
   {id: 1, discount_percent:0, quantity:6, title:"Tier 1"},
   {id: 2, discount_percent:5, quantity:8, title:"Tier 2"},
   {id: 3, discount_percent:10, quantity:10, title:"Tier 3"},
   {id: 4, discount_percent:12, quantity:12, title:"Tier 4"},
]
function calculateDiscount(){
   const ordersQuanity = 10;
   const bestTier = tiers.reduce((a, tier) => (
     tier.quantity <= ordersQuanity && (!a || tier.discount_percent > a.discount_percent)
       ? tier
       : a
   ), null) || tiers[0]; // alternate with the first element of the array
   // if you want to default to that tier even if the quantity isn't sufficient
   console.log(bestTier);
}
calculateDiscount();

If you happen to be able to assume that every increased discount_percent comes with a larger quantity, and the array is sorted, you can use .find if you reverse the array first (so that the items with the greatest discount_percent are iterated over first).

let tiers = [
   {id: 1, discount_percent:0, quantity:6, title:"Tier 1"},
   {id: 2, discount_percent:5, quantity:8, title:"Tier 2"},
   {id: 3, discount_percent:10, quantity:10, title:"Tier 3"},
   {id: 4, discount_percent:12, quantity:12, title:"Tier 4"},
];
const tiersReversed = [...tiers].reverse();
function calculateDiscount(){
   const ordersQuanity = 10;
   const tier = tiersReversed
     .find((_tier) => _tier.quantity <= ordersQuanity)
      || tiers[0]; // alternate with the first element of the array
   // if you want to default to that tier even if the quantity isn't sufficient
   console.log(tier);
}
calculateDiscount();

The snippet works just as well for the dataset in the edited question.

let tiers = [
   {id: 1, discount_percent:0, quantity:6, title:"Tier 1"},
   {id: 2, discount_percent:5, quantity:8, title:"Tier 2"},
   {id: 3, discount_percent:10, quantity:10, title:"Tier 3"},
   {id: 4, discount_percent:12, quantity:12, title:"Tier 4"},
   {id: 5, discount_percent:14, quantity:14, title:"Tier 5"},
   {id: 6, discount_percent:20, quantity:16, title:"Tier 6"},
   {id: 7, discount_percent:40, quantity:18, title:"Tier 7"},
   {id: 8, discount_percent:50, quantity:50, title:"Tier 8"},
]
function calculateDiscount(ordersQuanity){
   const bestTier = tiers.reduce((a, tier) => (
     tier.quantity <= ordersQuanity && (!a || tier.discount_percent > a.discount_percent)
       ? tier
       : a
   ), null) || tiers[0]; // alternate with the first element of the array
   // if you want to default to that tier even if the quantity isn't sufficient
   console.log(bestTier);
}
calculateDiscount(10);
calculateDiscount(20);