Constructing strings from character matrices and numeric arrays in JavaScript

Question

We need to write a JavaScript function that accepts an n * n matrix of string characters and an array of integers (positive and unique).

Our function should construct a string consisting of characters that exist at a 1-based index in a numeric array.

Character matrix-

[
   [‘a’, ‘b’, ‘c’, d’],
   [‘o’, ‘f’, ‘r’, ‘g’],
   [‘h’, ‘i’, ‘e’, ‘j’],
   [‘k’, ‘l’, ‘m’, n’]
];

Numeric array-

[1, 4, 5, 7, 11]

should return "adore" because these are the characters that occur at the 1-based index specified by the numeric array in the matrix .

Example

The following is the code-

Live demonstration

const arr = [
   ['a', 'b', 'c', 'd'],
   ['o', 'f', 'r', 'g'],
   ['h', 'i', 'e', 'j'],
   ['k', 'l', 'm', 'n']
];
const pos = [1, 4, 5, 7, 11];
const buildString = (arr = [], pos = []) => {
   const flat = [];
   arr.forEach(sub => {
      flat.push(...sub);
   });
   let res = '';
   pos.forEach(num => {
      res += (flat[num - 1] || '');
   });
   return res;
};
console.log(buildString(arr, pos));

Output

The following is the console output-

adore

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