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Nested loops

Loops can be nested into each other. In case of a for loop you have to use two different loop variables. 
let a = [[1, 7, 3], [2, 8, 5], [9, 0, 4]];
let sum = 0;
for (let i = 0; i < 3; i++) {
  for (let j = 0; j < 3; j++) {
    sum = sum + a[i][j];
  }
}
The code snippet calculates the sum of all elements of a two-dimensional array.

Exercise

Write a function sum that calculates the sum of all elements of a two-dimensional array.

Example: sum([[1, 2], [3]]) should return 6.

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