📝 TypeScript Coding Challenge

Welcome to your TypeScript interview challenge!
In this exercise you will choose TWO out of four functions to implement. Each function covers core string and array manipulation skills in TypeScript. You'll get instant feedback via tests, and can also run your code with logs to aid debugging.

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📂 Project Structure

src
├── functions
│   ├── countOccurrences.ts
│   ├── findDuplicatePositions.ts
│   ├── findDuplicates.ts
│   └── symmetricDifference.ts
├── tests
│   ├── countOccurrences.spec.ts
│   ├── findDuplicatePositions.spec.ts
│   ├── findDuplicates.spec.ts
│   └── symmetricDifference.spec.ts
├── run.ts
└── testData.json

🔍 Functions

Pick any two of the following to implement:

1. /**
    * Counts the number of times a subString appears in a string (case-insensitive).
    * @param str The string to check
    * @param subString The subString to count
    * @returns The number of times the subString appears in the string
    */
   export function countOccurrences(str: string, subString: string): number {}

2. /**
    * Finds the positions of all duplicate numbers in an array.
    * @param arr The array to check for duplicates
    * @returns A map of all numbers that appear more than once in the array
    */
   export function findDuplicatePositions(arr: number[]): Record<number, number[]> {}

3. /**
    * Finds all duplicate characters in a string (case-sensitive)
    * @param str The string to check for duplicates
    * @returns An array containing all characters that appear more than once
    */
   export function findDuplicates(str: string): string[] {}

4. /**
    * Computes the symmetric difference of two arrays.
    * Returns all elements that appear in exactly one of the two arrays (but not in both).
    * @param arrayOne First array
    * @param arrayTwo Second array
    * @returns New array of elements that appear in exactly one of the two arrays (but not in both).
    */
   export function symmetricDifference(arrayOne: number[], arrayTwo: number[]): number[] {}

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🚀 Getting Started

1. Pick the TWO functions you would like to implement

2. Install dependencies

   npm install

3. Implement your chosen functions Edit the stub files in src/functions.

4. Run tests for instant feedback Each function has its own test script:

   npm run test:co   # countOccurrences
   npm run test:sd   # symmetricDifference
   npm run test:fd   # findDuplicates
   npm run test:fdp  # findDuplicatePositions

   These commands use Vitest to run only the tests for the function you're working on.

5. View console logs (optional) If you sprinkle console.log statements in your code, use the run scripts to see them in action:

   npm run start:co   # countOccurrences
   npm run start:sd   # symmetricDifference
   npm run start:fd   # findDuplicates
   npm run start:fdp  # findDuplicatePositions

   Test output is kept clean; logs appear only when you explicitly run via run.ts.

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📊 Test Data

All inputs and expected outputs live in src/testData.json. Inspect this file to understand boundary conditions and sample cases.

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💡 Tips & Best Practices

• Embrace TypeScript types—define clear parameter and return types.
• Handle edge cases (empty strings/arrays, unexpected input).
• Keep functions pure; avoid side-effects.
• Write clean, readable code: small helpers and comments are welcome.
• Run tests frequently to confirm you're on the right track.

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🙋🏻‍♂️ Questions

• Questions are welcome
• We are happy to talk through the problem with you