Check if an array is sorted in Javascript...

check_sorted.js

/*
 * check the array is sorted
 * return: if positive ->  1
 *         if negative -> -1
 *         not sorted  ->  0
 */
Array.prototype.isSorted = function() {
  return (function(direction) {
    return this.reduce(function(prev, next, i, arr) {
      if (direction === undefined)
        return (direction = prev <= next ? 1 : -1) || true;
      else
        return (direction + 1 ?
          (arr[i-1] <= next) : 
          (arr[i-1] >  next));
    }) ? Number(direction) : false;
  }).call(this);
}

var arr = [3,2,1,0];
arr.isSorted();

function isSorted(array) {
  const limit = array.length - 1;
  for (let i = 0; i < limit; i++) {
    const current = array[i], next = array[i + 1];
    if (current > next) { return false; }
  }
  return true;
}

Inspired by @theQuazz

!![-Infinity,-1,0,2,3,Infinity].reduce((memo, item) => 'boolean' !== typeof memo && item >= memo && item) // true

!![-Infinity,-1,0,2,3,Infinity,2].reduce((memo, item) => 'boolean' !== typeof memo && item >= memo && item) // false

!![0,2,3,4].reduce((memo, item) => 'boolean' !== typeof memo && item >= memo && item) // true

!![0,2,3,4,2].reduce((memo, item) => 'boolean' !== typeof memo && item >= memo && item) // false

ES6

let arr = [-Infinity,-1,0,2,3,Infinity]

arr.slice(1).every((item, i) => arr[i] <= item) // true


arr = [-Infinity,-1,0,2,3,Infinity,2]

arr.slice(1).every((item, i) => arr[i] <= item) // false

function isSorted(boxNumbers) {
  for (let i = 1; i < boxNumbers.length; i++) {
    if (boxNumbers[i - 1] > boxNumbers[i]) {
      return false;
    }
  }
  return true;
}

(formatted by @yorkie)