Last digit of a large number.js

/*
Description:
Define a function that takes in two non-negative integers a and b and returns the last decimal digit of a^b. Note that a and b may be very large!

For example, the last decimal digit of 9^7 is 9, since 9^7 = 4782969. The last decimal digit of (2^200)^(2^300), which has over 10^92 decimal digits, is 6. Also, please take 0^0 to be 1.

You may assume that the input will always be valid.

Examples
lastDigit("4", "1")            // returns 4
lastDigit("4", "2")            // returns 6
lastDigit("9", "7")            // returns 9    
lastDigit("10","10000000000")  // returns 0
Remarks
JavaScript, C++, R, PureScript
Since these languages don't have native arbitrarily large integers, your arguments are going to be strings representing non-negative integers instead.
*/

var lastDigit = function(str1, str2){  
  return +!+str2 || Math.pow(str1.slice(-1) % 10, str2.slice(-2) % 4 || 4) % 10
}