how come I cannot do a Math.max on an array that has been sliced out of the another array-CodePudding

I am looking for a max number in each subarrays of an array. But when I did that it give me NaN. Why?

Let me outline what I've found. For context, I perform these tests in the console of Google Chrome.

Original array:

let arr = [[4, 5, 1, 3], [13, 27, 18, 26]];

I slice out the 1st subarray and assigned to arr1.

let arr1 = arr.slice(0, 1);
arr1;
 [4, 5, 1, 3]
arr1[0];   //this is to see if I can get value at index 0
 [4, 5, 1, 3]
arr1.length;
1

When I perform the Math.max on arr1, I get NaN.

Math.max(...arr1);
NaN

When I couldn't get the Math.max value, I thought arr1 wasn't an array so I test to see if it is an array:

Array.isArray(arr1)
true

I was able to get the max number from the subarray using the original arry. My question is why I cannot get the Math.max on arr1, the array that sliced out of the original?

CodePudding user response：

arr1 is an array of arrays, not an array of numbers. You can flatten the array before using Math.max. (Note: if you just want the ith subarray, just use arr[i] instead of slice. slice is only useful when you want multiple consecutive elements from an array.)

let arr = [[4, 5, 1, 3], [13, 27, 18, 26]];
let arr1 = arr.slice(0, 1);
console.log(JSON.stringify(arr1));
let max = Math.max(...arr1.flat());
console.log(max);

CodePudding user response：

Your arr1 isn't [4, 5, 1, 3], it's [[4, 5, 1, 3]] — an array containing a single element, which is an array of numbers. slice creates an array containing the elements you've selected from the original array (just the first one in your case). That first element in arr is an array, so you end up with an array containing that array.

If you want to use arr1 as you've shown, just use arr[0], not arr.slice(0, 1):

let arr = [[4, 5, 1, 3], [13, 27, 18, 26]];
let arr1 = arr[0];
console.log(Math.max(...arr1));