diff --git a/Advanced-Techniques/MoAlgorithm.js b/Advanced-Techniques/MoAlgorithm.js
new file mode 100644
index 0000000000..496f1b8197
--- /dev/null
+++ b/Advanced-Techniques/MoAlgorithm.js
@@ -0,0 +1,147 @@
+/**
+ * Mo's Algorithm for answering range queries efficiently
+ * @module MoAlgorithm
+ *
+ * Convention: queries are [left, right) — right is exclusive.
+ */
+
+export class Query {
+  /**
+   * @param {number} left - inclusive
+   * @param {number} right - exclusive
+   * @param {number} index - original index of the query
+   */
+  constructor(left, right, index) {
+    this.left = left
+    this.right = right
+    this.index = index
+  }
+}
+
+class QueryComparator {
+  constructor(blockSize) {
+    this.blockSize = blockSize
+  }
+
+  compare(a, b) {
+    const blockA = Math.floor(a.left / this.blockSize)
+    const blockB = Math.floor(b.left / this.blockSize)
+    if (blockA !== blockB) {
+      return blockA - blockB
+    }
+    return blockA % 2 === 0 ? a.right - b.right : b.right - a.right
+  }
+}
+
+export class MoAlgorithm {
+  /**
+   * @param {Array} arr
+   * @param {Array<Query>} queries
+   * @param {Function} addElement - called with arr[index]
+   * @param {Function} removeElement - called with arr[index]
+   * @param {Function} getResult - returns current result
+   */
+  constructor(arr, queries, addElement, removeElement, getResult) {
+    this.arr = arr
+    this.queries = queries
+    this.addElement = addElement
+    this.removeElement = removeElement
+    this.getResult = getResult
+    this.blockSize = Math.max(1, Math.floor(Math.sqrt(arr.length)))
+    this.comparator = new QueryComparator(this.blockSize)
+  }
+
+  validateQueries() {
+    for (const q of this.queries) {
+      if (
+        !Number.isInteger(q.left) ||
+        !Number.isInteger(q.right) ||
+        q.left < 0 ||
+        q.right < 0 ||
+        q.left > q.right ||
+        q.right > this.arr.length
+      ) {
+        throw new RangeError(
+          `Invalid query bounds: [${q.left}, ${q.right}) for array length ${this.arr.length}`
+        )
+      }
+    }
+  }
+
+  processQueries() {
+    // Validate input ranges first (right is allowed to be equal to arr.length)
+    this.validateQueries()
+
+    const sorted = [...this.queries].sort((a, b) =>
+      this.comparator.compare(a, b)
+    )
+    const results = new Array(this.queries.length)
+    let currentLeft = 0
+    let currentRight = -1 // current range is empty
+
+    for (const q of sorted) {
+      const left = q.left
+      const rightExclusive = q.right
+      const targetRight = rightExclusive - 1 // inclusive target index
+
+      // expand right
+      while (currentRight < targetRight) {
+        currentRight++
+        // safe: currentRight is guaranteed in [0, arr.length-1] by validation
+        this.addElement(this.arr[currentRight])
+      }
+
+      // move left leftwards (expand)
+      while (currentLeft > left) {
+        currentLeft--
+        this.addElement(this.arr[currentLeft])
+      }
+
+      // shrink right
+      while (currentRight > targetRight) {
+        this.removeElement(this.arr[currentRight])
+        currentRight--
+      }
+
+      // shrink left
+      while (currentLeft < left) {
+        this.removeElement(this.arr[currentLeft])
+        currentLeft++
+      }
+
+      results[q.index] = this.getResult()
+    }
+
+    return results
+  }
+}
+
+export class UniqueElementsCounter {
+  constructor() {
+    this.frequency = new Map()
+    this.uniqueCount = 0
+  }
+
+  addElement(element) {
+    const count = this.frequency.get(element) || 0
+    this.frequency.set(element, count + 1)
+    if (count === 0) {
+      this.uniqueCount++
+    }
+  }
+
+  removeElement(element) {
+    const count = this.frequency.get(element) || 0
+    if (count <= 0) return
+    if (count === 1) {
+      this.frequency.delete(element)
+      this.uniqueCount--
+    } else {
+      this.frequency.set(element, count - 1)
+    }
+  }
+
+  getResult() {
+    return this.uniqueCount
+  }
+}
diff --git a/Advanced-Techniques/tests/MoAlgorithm.test.js b/Advanced-Techniques/tests/MoAlgorithm.test.js
new file mode 100644
index 0000000000..3c2251db6b
--- /dev/null
+++ b/Advanced-Techniques/tests/MoAlgorithm.test.js
@@ -0,0 +1,38 @@
+import { MoAlgorithm, Query, UniqueElementsCounter } from '../MoAlgorithm.js'
+import { describe, it, expect } from 'vitest'
+
+describe('MoAlgorithm', () => {
+  it('should process range queries correctly', () => {
+    const arr = [1, 2, 3, 2, 1, 4, 5]
+    // right is exclusive: to cover indices 0..2 use right=3, 1..4 use right=5, 2..5 use right=6
+    const queries = [new Query(0, 3, 0), new Query(1, 5, 1), new Query(2, 6, 2)]
+
+    const counter = new UniqueElementsCounter()
+    const processor = new MoAlgorithm(
+      arr,
+      queries,
+      (element) => counter.addElement(element),
+      (element) => counter.removeElement(element),
+      () => counter.getResult()
+    )
+
+    const results = processor.processQueries()
+    expect(results).toEqual([3, 3, 4])
+  })
+
+  it('should handle empty array', () => {
+    const arr = []
+    const queries = [new Query(0, 0, 0)]
+    const counter = new UniqueElementsCounter()
+    const processor = new MoAlgorithm(
+      arr,
+      queries,
+      counter.addElement.bind(counter),
+      counter.removeElement.bind(counter),
+      counter.getResult.bind(counter)
+    )
+
+    const results = processor.processQueries()
+    expect(results).toEqual([0])
+  })
+})