added BasicPrefixSum code

Author: mbatwc113114

PrefixSum/BasicPrefixSum.js

@@ -0,0 +1,25 @@
+/**
+ * Computes the prefix sum (cumulative sum) of an array of numbers.
+ * For example, given [1, 2, 3], returns [1, 3, 6].
+ *
+ * @param {number[]} arr - The input array of numbers.
+ * @returns {number[]} - The prefix sum array.
+ * @throws {TypeError} - If input is not an array of numbers.
+ */
+function basicPrefixSum(arr) {
+    // Validate input: must be an array of numbers
+    if (!Array.isArray(arr) || !arr.every(x => typeof x === 'number')) {
+        throw new TypeError('Input must be an array of numbers');
+    }
+
+    const prefix = [];
+    // Use reduce to calculate cumulative sum and store in prefix array
+    arr.reduce((acc, val, index) => {
+        prefix[index] = acc + val; // Store current sum at index
+        return prefix[index];      // Pass current sum to next iteration
+    }, 0);
+    return prefix;
+}
+
+// Export the function for use in other modules
+module.exports = basicPrefixSum;

PrefixSum/test/BasicPrefixSum.test.js

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+import basicPrefixSum from '../BasicPrefixSum'  
+
+describe('basicPrefixSum', () => {
+  it('computes correct prefix sums for normal arrays', () => {
+    expect(basicPrefixSum([1, 2, 3, 4])).toEqual([1, 3, 6, 10])
+    expect(basicPrefixSum([0, 0, 0])).toEqual([0, 0, 0])
+    expect(basicPrefixSum([-1, 5, -3])).toEqual([-1, 4, 1])
+    expect(basicPrefixSum([100])).toEqual([100]) // single element array
+  })
+
+  it('returns an empty array for empty input', () => {
+    expect(basicPrefixSum([])).toEqual([])
+  })
+
+  it('throws TypeError for invalid inputs', () => {
+    expect(() => basicPrefixSum('not an array')).toThrow(TypeError)
+    expect(() => basicPrefixSum([1, 2, 'x'])).toThrow(TypeError)
+    expect(() => basicPrefixSum([{}, 3])).toThrow(TypeError)
+    expect(() => basicPrefixSum(null)).toThrow(TypeError)
+    expect(() => basicPrefixSum(undefined)).toThrow(TypeError)
+  })
+})