diff --git a/immutable-data-cost/README.md b/immutable-data-cost/README.md
new file mode 100644
index 0000000..6647544
--- /dev/null
+++ b/immutable-data-cost/README.md
@@ -0,0 +1,15 @@
+# Immutability cost
+
+## Immutable methods
+
+- Object.values/keys/entries
+- concat
+- rest/spread `...`
+- slice
+- filter/map
+
+## Mutable methods
+
+- Object.assign
+- splice
+- push
diff --git a/immutable-data-cost/code-expamples.ts b/immutable-data-cost/code-expamples.ts
new file mode 100644
index 0000000..1f6a35b
--- /dev/null
+++ b/immutable-data-cost/code-expamples.ts
@@ -0,0 +1,86 @@
+function filterNumbersNotOptimal(array: number[]) {
+  return array
+    .filter((item) => item % 2 === 0) // O(N) cpu + memory
+    .map((item) => item * 2) // O(N) cpu + memory
+    .filter((item) => item >= 1_000); // O(N) cpu + memory
+}
+
+function filterNumbersOptimal(array: number[]) {
+  const filteredArray: number[] = [];
+
+  array.forEach((item) => {
+    if (item % 2 === 1) {
+      return;
+    }
+
+    const multiplied = item * 2;
+
+    if (multiplied < 1_000) {
+      return;
+    }
+
+    filteredArray.push(item);
+  });
+
+  return filteredArray;
+}
+
+function flattenOneNotOptimal<T>(arrays: T[][]) {
+  return arrays.reduce((result, array) => result.concat(array), []);
+}
+
+function flattenOneOptimal<T>(arrays: T[][]) {
+  return arrays.reduce((result, array) => {
+    // return result.push(...array);
+    return result.push.apply(result, array);
+  }, []);
+}
+
+function updateObjectValuesNotOptimal(object: Record<string, number>) {
+  return Object.fromEntries(
+    Object.entries(object).map(([key, value]) => [key, value * 2])
+  );
+}
+
+function updateObjectValuesOptimal(object: Record<string, number>) {
+  const newObject: Record<string, number> = {};
+
+  for (const key in object) {
+    newObject[key] = object[key] * 2;
+  }
+
+  return newObject;
+}
+
+function sumTwoNotOptimal(array: number[], sum: number) {
+  //   const set = new Set([...array]);
+  const set = new Set(array);
+
+  for (let i = 0, l = array.length; i < l; i++) {
+    const num = array[i];
+    const difference = sum - num;
+
+    if (set.has(difference)) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+function sumTwoOptimal(array: number[], sum: number) {
+  const set = new Set();
+
+  for (let i = 0, l = array.length; i < l; i++) {
+    const num = array[i];
+    const difference = sum - num;
+
+    if (set.has(difference)) {
+      return true;
+    }
+
+    set.add(num);
+  }
+
+  return false;
+}
diff --git a/immutable-data-cost/methods-examples.ts b/immutable-data-cost/methods-examples.ts
new file mode 100644
index 0000000..fdf68f3
--- /dev/null
+++ b/immutable-data-cost/methods-examples.ts
@@ -0,0 +1,63 @@
+const array = [1, 2, 3, 4];
+const object = {
+  a: 1,
+  b: 2,
+  c: "asdf",
+};
+
+// ==========================
+// COPY
+// ==========================
+const copy1 = {
+  ...object,
+};
+
+// mutates 1st object
+const copy2 = Object.assign({}, object);
+
+const copy3 = [...array];
+const copy4 = array.slice();
+const copy5 = array.concat([]);
+
+function immutableCopyArray<T>(array: T[]) {
+  const newArray: T[] = [];
+
+  for (let i = 0, l = array.length; i < l; i++) {
+    newArray[i] = array[i];
+  }
+
+  return newArray;
+}
+
+function immutableCopyObject<T>(object: Record<string, T>) {
+  const newRecord = {};
+
+  for (const key in object) {
+    newRecord[key] = object[key];
+  }
+
+  return newRecord;
+}
+
+// ==========================
+// iterate
+// ==========================
+const filtered = array.filter((item) => item > 0);
+const mapped = array.map((item) => item * 1);
+
+Object.keys(object).forEach((key) => {
+  console.log(key, object[key]);
+});
+
+Object.values(object).forEach((value) => {
+  console.log(value);
+});
+
+Object.entries(object).forEach(([key, value]) => {
+  console.log(key, value);
+});
+
+for (const key in object) {
+  const value = object[key];
+  console.log(key, value);
+}