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function largestOfFour(arr) {
 // Create new array to use later
  var largestArray = [];
  // This first array runs through each sub array
  for (var i = 0; i < arr.length; i++ ) { 
    // Sets to zero every time it iterates through this for loop
    var largestNumber = 0; 
    // Runs through contents of each sub array
    for (var j = 0; j < arr.length; j++){ 
      // Picks out largest number
      if (largestNumber < arr[i][j]) { 
        largestNumber = arr[i][j];
      }
    }
    // Pushes largest number from each sub array to the array created at the start
    largestArray.push(largestNumber); 
  }
  
  return largestArray;
}

// Example call
largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);