Return Largest Numbers in Arrays
Problem Explanation
You will get an array that contains sub arrays of numbers and you need to return an array with the largest number from each of the sub arrays.
Hints
Hint 1
You will get an array that contains sub arrays of numbers and you need to return an array with the largest number from each of the sub arrays. You will need to keep track of the array with the answer and the largest number of each subarray.
Hint 2
You can work with multidimensional arrays by Array[Index][SubIndex]
Hint 3
Pay close attention to the timing of the storing of variables when working with loops
Solutions
Solution 1 (Click to Show/Hide)
(Procedural approach)
function largestOfFour(arr) {
let results = [];
for (let i = 0; i < arr.length; i++) {
let largestNumber = arr[i][0];
for (let j = 1; j < arr[i].length; j++) {
if (arr[i][j] > largestNumber) {
largestNumber = arr[i][j];
}
}
results[i] = largestNumber;
}
return results;
}
Code Explanation
 Create a variable to store the results as an array.
 Create an outer loop to iterate through the outer array.
 Create a second variable to hold the largest number and initialise it with the first number. This must be outside an inner loop so it wonâ€™t be reassigned until we find a larger number.
 Create said inner loop to work with the subarrays.
 Check if the element of the sub array is larger than the currently stored largest number. If so, then update the number in the variable.
 After the inner loop, save the largest number in the corresponding position inside of the
results
array.  And finally return said array.
Relevant Links
Solution 2 (Click to Show/Hide)
(Declarative approach)
function largestOfFour(arr) {
return arr.map(function(group) {
return group.reduce(function(prev, current) {
return current > prev ? current : prev;
});
});
}
Code Explanation
 we map all items within the main array to a new array using
Array.prototype.map()
and return this array as the final result  within each inner array, we reduce its contents down to a single value using
Array.prototype.reduce()
 the callback function passed to the reduce method takes the previous value and the current value and compares the two values
 if the current value is higher than the previous value we set it as the new previous value for comparison with the next item within the array or returns it to the map method callback if itâ€™s the last item
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Solution 3 (Click to Show/Hide)
(Declarative approach)
function largestOfFour(arr) {
return arr.map(Function.apply.bind(Math.max, null));
}
Code Explanation
TL;DR: We build a special callback function (using the Function.bind
method), that works just like Math.max
but also has Function.prototype.apply
's ability to take arrays as its arguments.
 We start by mapping through the elements inside the main array. Meaning each one of the inner arrays.
 Now the need a callback function to find the max of each inner array provided by the map.
So we want to create a function that does the work of Math.max
and accepts input as an array (which by it doesnâ€™t by default).
In other words, it would be really nice and simple if this worked by itself:
Math.max([9, 43, 20, 6]); // Resulting in 43
Alas, it doesnâ€™t.
 To do the work of accepting arguments in the shape of an array, there is this
Function.prototype.apply
method, but it complicates things a bit by invoking the context function.
i.e. Math.max.apply(null, [9, 43, 20, 6]);
would invoke something like a Max.max
method. What weâ€™re looking forâ€¦ almost.
Here weâ€™re passing null
as the context of the Function.prototype.apply
method as Math.max
doesnâ€™t need any context.
 Since
arr.map
expects a callback function, not just an expression, we create a function out of the previous expression by using theFunction.bind
method.  Since,
Function.prototype.apply
is a static method of the sameFunction
object, we can callFunction.prototype.bind
onFunction.prototype.apply
i.e.Function.prototype.apply.bind
.  Now we pass the context for the
Function.prototype.apply.bind
call (in this case we wantMath.max
so we can gain its functionality).  Since the embedded
Function.prototype.apply
method will also require a context as itâ€™s 1st argument, we need to pass it a bogus context.
So, we pass
null
as the 2nd param toFunction.prototype.apply.bind
which gives a context to theMath.max
method. 
Since,
Math.max
is independent of any context, hence, it ignores the bogus context given byFunction.prototype.apply
method call. 
Thus, our
Function.prototype.apply.bind(Math.max, null)
makes a new function accepting thearr.map
values i.e. the inner arrays.

Relevant Links
Solution 4 (Click to Show/Hide)
(Recursive approach)
function largestOfFour(arr, finalArr = []) {
return !arr.length
? finalArr
: largestOfFour(arr.slice(1), finalArr.concat(Math.max(...arr[0])))
}