> # Welcome to the TypeScript community

>
> ### Describe your change:
> * [x]  Add an algorithm?
> * [ ]  Fix a bug or typo in an existing algorithm?
> * [ ]  Documentation change?
>
> ### Checklist:
> * [x]  I have read [CONTRIBUTING.md](https://github.com/TheAlgorithms/Javascript/blob/master/CONTRIBUTING.md).
> * [x]  This pull request is all my own work -- I have not plagiarized.
> * [x]  I know that pull requests will not be merged if they fail the automated tests.
> * [x]  This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
> * [x]  All new JavaScript files are placed inside an existing directory.
> * [x]  All filenames should use the UpperCamelCase (PascalCase) style. There should be no spaces in filenames.
>   **Example:**`UserProfile.js` is allowed but `userprofile.js`,`Userprofile.js`,`user-Profile.js`,`userProfile.js` are not
> * [x]  All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
> * [x]  If this pull request resolves one or more open issues then the commit message contains `Fixes: #{$ISSUE_NO}`.

Author: Owais28

.gitignore

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+# dependencies
+/node_modules
+/temp
+
+# misc
+.DS_Store
+.env.local
+.env.development.local
+.env.test.local
+.env.production.local
+
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# intelliJ workspace folder
+.idea

.husky/pre-commit

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+#!/usr/bin/env sh
+. "$(dirname -- "$0")/_/husky.sh"
+
+npm test

Data-structures/QuickSelect.ts

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+/**
+ * [QuickSelect](https://www.geeksforgeeks.org/quickselect-algorithm/) is an algorithm to find the kth smallest number
+ *
+ * Notes:
+ * - QuickSelect is related to QuickSort, thus has optimal best and average case (O(n)) but unlikely poor worst case (O(n^2))
+ * - This implementation uses randomly selected pivots for better performance
+ * ----
+ * @complexity : O(n) (on average )
+ * @complexity : O(n^2) (worst case)
+ * ----
+ * @param items array
+ */
+
+const QuickSelect = (items: Array<number>, kth: number): number => {
+  // eslint-disable-line no-unused-vars
+  if (kth < 1 || kth > items.length) {
+    throw new RangeError("Index Out of Bound");
+  }
+
+  return RandomizedSelect(items, 0, items.length - 1, kth);
+};
+
+/**
+ * @param items
+ * @param left
+ * @param right
+ * @param i
+ * @returns number
+ */
+const RandomizedSelect = (
+  items: Array<number>,
+  left: number,
+  right: number,
+  i: number
+): number => {
+  if (left === right) return items[left];
+
+  const pivotIndex = RandomizedPartition(items, left, right);
+  const k = pivotIndex - left + 1;
+
+  if (i === k) return items[pivotIndex];
+  if (i < k) return RandomizedSelect(items, left, pivotIndex - 1, i);
+
+  return RandomizedSelect(items, pivotIndex + 1, right, i - k);
+};
+/**
+ * 
+ * @param items 
+ * @param left 
+ * @param right 
+ * @returns 
+ */
+const RandomizedPartition = (
+  items: Array<number>,
+  left: number,
+  right: number
+): number => {
+  const rand = getRandomInt(left, right);
+  Swap(items, rand, right);
+  return Partition(items, left, right);
+};
+/**
+ * 
+ * @param items 
+ * @param left 
+ * @param right 
+ * @returns 
+ */
+const Partition = (items: Array<number>, left: number, right: number) : number => {
+  const x = items[right];
+  let pivotIndex = left - 1;
+
+  for (let j = left; j < right; j++) {
+    if (items[j] <= x) {
+      pivotIndex++;
+      Swap(items, pivotIndex, j);
+    }
+  }
+
+  Swap(items, pivotIndex + 1, right);
+
+  return pivotIndex + 1;
+};
+
+/**
+ * 
+ * @param min 
+ * @param max 
+ * @returns 
+ */
+const getRandomInt = (min : number, max : number) : number => {
+  return Math.floor(Math.random() * (max - min + 1)) + min;
+}
+
+
+/**
+ * 
+ * @param arr array
+ * @param x array element to swap
+ * @param y array element to swap
+ */
+const Swap = (arr : Array<number>, x : number, y : number) : void => {
+  [arr[x], arr[y]] = [arr[y], arr[x]];
+}
+
+export { QuickSelect };

Data-structures/test/QuickSelect.test.ts

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+import { QuickSelect } from '../QuickSelect'
+
+describe('QuickSelect tests', () => {
+  it('should return the only element of a list of length 1', () => {
+    // Test a mix of number types (i.e., positive/negative, numbers with decimals, fractions)
+    expect(QuickSelect([100], 1)).toEqual(100)
+    expect(QuickSelect([-23], 1)).toEqual(-23)
+    expect(QuickSelect([2007.102], 1)).toEqual(2007.102)
+    expect(QuickSelect([0.9], 1)).toEqual(0.9)
+    expect(QuickSelect([-0.075], 1)).toEqual(-0.075)
+    expect(QuickSelect([0], 1)).toEqual(0)
+    expect(QuickSelect([1], 1)).toEqual(1)
+  })
+
+  it('should throw an Error when k is greater than the length of the list', () => {
+    expect(() => QuickSelect([100, 2], 5)).toThrow('Index Out of Bound')
+  })
+
+  it('should throw an Error when k is less than 1', () => {
+    expect(() => QuickSelect([100, 2], 0)).toThrow('Index Out of Bound')
+    expect(() => QuickSelect([100, 2], -1)).toThrow('Index Out of Bound')
+  })
+
+  describe('varieties of list composition', () => {
+    it('should return the kth smallest element of a list that is in increasing order', () => {
+      expect(QuickSelect([10, 22, 33, 44, 55], 1)).toEqual(10)
+      expect(QuickSelect([10, 22, 33, 44, 55], 2)).toEqual(22)
+      expect(QuickSelect([10, 22, 33, 44, 55], 3)).toEqual(33)
+      expect(QuickSelect([10, 22, 33, 44, 55], 4)).toEqual(44)
+      expect(QuickSelect([10, 22, 33, 44, 55], 5)).toEqual(55)
+    })
+
+    it('should return the kth smallest element of an input list that is in decreasing order', () => {
+      expect(QuickSelect([82, 33.12, 4.0, 1], 1)).toEqual(1)
+      expect(QuickSelect([82, 33.12, 4.0, 1], 2)).toEqual(4.0)
+      expect(QuickSelect([82, 33.12, 4.0, 1], 2)).toEqual(4)
+      expect(QuickSelect([82, 33.12, 4.0, 1], 3)).toEqual(33.12)
+      expect(QuickSelect([82, 33.12, 4.0, 1], 4)).toEqual(82)
+    })
+
+    it('should return the kth smallest element of an input list that is no particular order', () => {
+      expect(QuickSelect([123, 14231, -10, 0, 15], 3)).toEqual(15)
+      expect(QuickSelect([0, 15, 123, 14231, -10], 3)).toEqual(15)
+      expect(QuickSelect([-10, 15, 123, 14231, 0], 3)).toEqual(15)
+      expect(QuickSelect([14231, 0, 15, 123, -10], 3)).toEqual(15)
+      expect(QuickSelect([14231, 0, 15, -10, 123], 3)).toEqual(15)
+    })
+  })
+})

babel.config.js

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+module.exports = {
+    presets: [
+        ['@babel/preset-env', { targets: { node: 'current' } }],
+        '@babel/preset-typescript'],
+
+};

jest.config.ts

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+import type { Config } from "@jest/types";
+// Sync object
+const config: Config.InitialOptions = {
+  verbose: true,
+  transform: {
+    "^.+\\.tsx?$": "ts-jest",
+  },
+};
+export default config;