numericFilter.md

Numeric Filters in Express + Mongoose (Deep Explanation)

🎯 INPUT (from the URL)

numericFilters=price>30,rating<4.5

So in Express:

numericFilters === "price>30,rating<4.5"

---

🧩 THE CODE WE ARE ANALYZING

if (numericFilters) {
  const operatorMap = {
    '>': '$gt',
    '>=': '$gte',
    '=': '$eq',
    '<': '$lt',
    '<=': '$lte',
  };

  const regEx = /\b(<|>|<=|>=|=)\b/g;
  let filters = numericFilters.replace(
    regEx,
    (match) => `-${operatorMap[match]}-`
  );

  const options = ['price', 'rating'];

  filters.split(',').forEach((item) => {
    const [field, operator, value] = item.split('-');
    if (options.includes(field)) {
      queryObject[field] = { [operator]: Number(value) };
    }
  });
}

---

🧠 STEP 1 — if (numericFilters)

✔ numericFilters exists
✔ Value = "price>30,rating<4.5"

➡ Code enters the block

---

🧠 STEP 2 — Operator Map

const operatorMap = {
  '>': '$gt',
  '>=': '$gte',
  '=': '$eq',
  '<': '$lt',
  '<=': '$lte',
};

📦 Memory now contains:

operatorMap['>'] → '$gt'
operatorMap['<'] → '$lt'

➡ This is your translator dictionary

---

🧠 STEP 3 — Regex Definition

const regEx = /\b(<|>|<=|>=|=)\b/g;

This regex finds comparison operators inside strings.

In:

price>30,rating<4.5

It finds:

• >
• <

---

🧠 STEP 4 — Replace Operators

let filters = numericFilters.replace(
  regEx,
  (match) => `-${operatorMap[match]}-`
);

🔍 Internal Execution

Original string:

price>30,rating<4.5

• > → -$gt-
• < → -$lt-

Final result:

filters === "price-$gt-30,rating-$lt-4.5"

🔥 THIS IS THE KEY TRANSFORMATION

---

🧠 STEP 5 — Allowed Fields

const options = ['price', 'rating'];

✔ Only safe fields allowed
✔ Prevents invalid or malicious queries

---

🧠 STEP 6 — Split by Comma

filters.split(',')

Result:

[
  "price-$gt-30",
  "rating-$lt-4.5"
]

---

🧠 STEP 7 — First Loop Iteration

item = "price-$gt-30"

Split:

["price", "$gt", "30"]

Destructuring:

field    = "price"
operator = "$gt"
value    = "30"

MongoDB query built:

queryObject.price = { $gt: 30 };

---

🧠 STEP 8 — Second Loop Iteration

item = "rating-$lt-4.5"

Split:

["rating", "$lt", "4.5"]

MongoDB query built:

queryObject.rating = { $lt: 4.5 };

---

🎯 FINAL RESULT

queryObject = {
  price: { $gt: 30 },
  rating: { $lt: 4.5 }
}

---

🚀 WHAT MONGODB RECEIVES

Product.find({
  price: { $gt: 30 },
  rating: { $lt: 4.5 }
});

📌 Meaning

Give me products where:

• price is greater than 30
• AND rating is less than 4.5