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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

Example:

`sum`

that calculates the sum of all elements of a two-dimensional array.Example:

`sum([[1, 2], [3]])`

should return `6`

.