add: solution❤️

Author: supercodestart

calculate-the-sum-of-the-numbers-in-a-nested-array/README.md

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+# Calculate the sum of the numbers in a nested array
+
+You need to calculate the sum of the numbers in an array. However, the array contains nested arrays, whose numbers you must also count. These nested arrays may also have further arrays recursively nested in them, and so on. You do not know how deep the nesting continues. If the list is empty, return zero.
+
+The arrays may also contain data that is not of the type number(`string`, `BigInt`, `object`, etc), which you should ignore.
+
+## Examples
+
+```javascript
+nestedSum([1, 2, [3, [4], 5, '6'], 6]); // should == 21
+nestedSum([2, [[[[[3]]]]]]); // should == 5
+nestedSum([]); // should == 0
+```
+
+## Execute
+
+```bash
+node solution.js
+```

calculate-the-sum-of-the-numbers-in-a-nested-array/solution.js

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+function nestedSum(arr) {
+  if (!arr || !Array.isArray(arr)) {
+    return 0;
+  }
+  return arr.flat(Infinity).reduce((prev, value) => prev + (typeof value === 'number' ? value : 0), 0);
+}
+
+(() => {
+  [
+    undefined,
+    null,
+    3,
+    'abc',
+    [1, 2, [3, [4], 5, '6'], 6],
+    [2, [[[[[3]]]]]],
+    [],
+    [1, 2, 3],
+    [1, 2, [4]],
+    [1, [2, [3], 4], [4, [[1]]]],
+    [[[[[[[[[[2]]]]]]]]], 3],
+  ].forEach((arr) => {
+    console.log(JSON.stringify(arr), '==', nestedSum(arr));
+  });
+})();