Explain this Javascript expression just like I'm five

parseInt() takes to arguments, a string and a radix. I usually pass just the string I want converting as a number, you are passing both the string and the radix!

map gives us the value and the index (and the actual array)

["10", "10", "10", "10"].map( (value,index) => ... )

By just doing .map(parseInt) parseInt is taking both the value and the index.

Basically the second iteration is parseInt('10',1) and returns NaN.

Finally, to test that that's what happens, the following would return "0: 10" "1: 10" "2: 10" "3: 10"

function myFunc(value, i){
  console.log(`${i}: ${value}`)
}
["10", "10", "10", "10"].map(myFunc)

["10", "10", "10", "10"].map(parseInt)  ==>  [10, NaN, 2, 3]

This is because from Array.prototype.map

map calls a provided callback function once for each element in an array, in order, and constructs a new array from the results.
callback is invoked with three arguments: the value of the element, the index of the element, and the Array object being traversed.

Also from parseInt()

Syntax: parseInt(string, radix);

So when this is executing it will return

[
    parseInt("10", 0),
    parseInt("10", 1),
    parseInt("10", 2),
    parseInt("10", 3)
]

take a list of strings that represent numbers and return a new list with all the strings converted to the numbers they represent, but they now have the data type of number