Check if array has element(s) from another array

NOTE: includes is not ES6, but ES2016 Mozilla docs. This will break if you transpile ES6 only.

You can use Array#every method(to iterate and check all element passes the callback function) with Array#includes method(to check number present in B).

A.every( e => B.includes(e) )

const A = [0, 1, 2],
  B = [2, 1, 0],
  C=[2, 1];

console.log(A.every(e => B.includes(e)));
console.log(A.every(e => C.includes(e)));
console.log(C.every(e => B.includes(e)));

A[0].includes(e) 
//^---index

Or using Array#indexOf method, for older browser.

A[0].indexOf(e) > -1 

Or in case you want to check at least one element present in the second array then you need to use Array#some method(to iterate and check at least one element passes the callback function).

A.some(e => B.includes(e) )

const A = [0, 1, 2],
  B = [2, 1, 0],
  C=[2, 1],D=[4];

console.log(A.some(e => B.includes(e)));
console.log(A.some(e => C.includes(e)));
console.log(C.some(e => B.includes(e)));
console.log(C.some(e => D.includes(e)));

Here's a self defined function I use to compare two arrays. Returns true if array elements are similar and false if different. Note: Does not return true if arrays are equal (array.len && array.val) if duplicate elements exist.

var first = [1,2,3];
var second = [1,2,3];
var third = [3,2,1];
var fourth = [1,3];
var fifth = [0,1,2,3,4];

console.log(compareArrays(first, second));
console.log(compareArrays(first, third));
console.log(compareArrays(first, fourth));
console.log(compareArrays(first, fifth));

function compareArrays(first, second){
    //write type error
    return first.every((e)=> second.includes(e)) && second.every((e)=> first.includes(e));
}

If the intent is to actually compare the array, the following will also account for duplicates

const arrEq = (a, b) => {
  if (a.length !== b.length) {
    return false
  }
  const aSorted = a.sort()
  const bSorted = b.sort()


  return aSorted
    .map((val, i) => bSorted[i] === val)
    .every(isSame => isSame)
}

Hope this helps someone :D