endZeros.js

/*
Challenge: Write a function that checks if all the zeros of an array are at the
end (an array with no zeros in it does have all its zeros at the end). An empty
array also has all its zeros at the end.

The function should run in O(n) time, and it's okay if it always runs in O(n).
The objective here is "elegance" (ease of reading the code), not
performance.

Inspired by this real world Ruby discussion:
    https://github.com/thewca/worldcubeassociation.org/pull/978/files#r85463024
*/

function areZerosAtEnd(arr) {
  let zeroSeen = false;
  return arr.reduce((validSoFar, currentValue) => {
    const isZero = currentValue === 0;
    zeroSeen = zeroSeen || isZero; 
    return validSoFar && (isZero || !zeroSeen);
  }, true);
}

console.assert(areZerosAtEnd([1,1,1,1,1]) === true, 'all non-zero should be true'); 		
console.assert(areZerosAtEnd([1,1,1,1,0]) === true, 'one zero at the end should be true');
console.assert(areZerosAtEnd([0,0,0]) === true, 'all zeros should be true');					
console.assert(areZerosAtEnd([2,0,0,0,0]) === true, 'a number with all zeros at the end should be true');	
console.assert(areZerosAtEnd([0,0,0,2,0]) === false, 'should be false. there are zeros after a non-zero.');		
console.assert(areZerosAtEnd([2,0,0,0,2,0]) === false, 'should be false. there is a zero after a non-zero');
console.assert(areZerosAtEnd([19,0,0,0,2,0,0]) === false, 'should be false. there are multiple zeros after a non-zero');