Function to find highest value from an array of values, not using `Math.max()`

Try this:

function maxVal(array){
    let indexOfMax = 0;
    for (let i = 0; i < array.length; i++){
        if (array[i] > array[indexOfMax]){
            indexOfMax = i;
        }
    }
    return array[indexOfMax];
}

Simply use Array.prototype.reduce, it's usually the smartest way for doing simple math operations like getting averages, ranges or min-max

function max(...values) {
  return values.reduce((prev, curr) => curr > prev ? curr : prev)
}
// or without the `...rest` parameter to use an array directly
function maxFromArr(values) {
  return values.reduce((prev, curr) => curr > prev ? curr : prev)
}

with this you have some clean and efficient code:

max(1, 2, 3, 8, 435, 32, 0) // => 432
// or for an array
const arr = [1, 2, 3, 72, 32, 0]
maxFromArray(arr) // => 74
// or
max(...arr) // => 74

Common Pitfall

When comparing numbers, usually always test for a positive (is instead of isNot)

Testing for a positive result (is the current larger ? use current, instead of is the current smaller ? use previous) will also automatically take care of any item that can't be turned into a proper number (comparing a number to NaN will always return false. eg. 12 < NaN and 12 > NaN both return false).

If you do it the other way around you might get a surprising result: (feel free to run this snippet in any playground)

function max (...values: number[]) {
  return values.reduce((prev, curr) => curr > prev ? curr : prev)
}

function naïveMax (...values: number[]) {
  // notice were testing for a negativ: is the current smaller ?
  // if it **isn't**, use the current
  return values.reduce((prev, curr) => curr < prev ? prev : curr)
}

const arr1 = [1, 21, 3, 12, 2]
console.log(max(...arr1)) // => 21
console.log(naïveMax(...arr1)) // => 21
// looking good so far

const arr2 = [89, -21, 32, {}, 342]
console.log(max(...arr2)) // => 342
console.log(naïveMax(...arr2)) // => 242
// looks still ok, despite an object being mixed in, but

const arr3 = [64, 2, 3, 95, 32, {}]
console.log(max(...arr3)) // => 95
console.log(naïveMax(...arr3)) // => {}
// whoops, the largest number suddenly is the object

here's min for completeness sake

function min(...values) {
  // again, test for a positive to filter `NaN`-values
  return values.reduce((prev, curr) => curr < prev ? curr : prev)
}

const getHighest = numbers.sort((a, b) => b-a)[0];