Checking JavaScript array equality using forEach

Question

I'm trying to check for array equality using forEach in JavaScript. This criteria is key, and the answer is not on Stack Overflow already.

The tests that are checking equality and expecting true are failing. I have a feeling I'm running into an issue to do with scope. Can someone please walk me through it?

function eql(arr1, arr2) {
  
  arr1.forEach((el) => {
    if (arr1[el] === arr2[el]) {
      return true;
    } 
  })
  return false
}

Here are the test results I want to change:

eql([], [])

Expected: true but got: false

eql(['a', 'b'], ['a', 'b'])

Expected: true but got: false

please add some examples. and results. btw, forEach does not return a result from an inner return. — Nina Scholz
– Nina Scholz, Commented Sep 29, 2020 at 7:33
return true within the callback doesn't do anything. It is returning *from the callback` not from eql. You either need to convert to using a conventional loop, change it so the result is outside the callback and only changed within, or change to .some()/.every() and return the result of that. — VLAZ
– VLAZ, Commented Sep 29, 2020 at 7:34
return true on the first match (if it would work, see comments above) doesn't tell you if all elements are equal. — Andreas
– Andreas, Commented Sep 29, 2020 at 7:35
@gorak every and most array methods skip holes. So, it will return true for [,,3] and [1,2,3] — adiga
– adiga, Commented Sep 29, 2020 at 7:42

adiga · Accepted Answer · 2020-09-29 13:25:07Z

1

This can be one of the many solutions

function eql(arr1, arr2) {
  let returnStatement = true;

  if (arr1.length !== arr2.length) {
    returnStatement = false;
  } else if (returnStatement) {
    arr1.forEach((el, index) => {
      if (el !== arr2[index]) {
        returnStatement = false;
      }
    });
  }

  return returnStatement;
}

console.log(eql([1, 2, 4], [1, 2, 4]))

edited Sep 29, 2020 at 13:25

adiga

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answered Sep 29, 2020 at 7:46

Ahmad Suddle

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Comments

Jamiec · Accepted Answer · 2020-09-29 07:40:26Z

1

There are a number of things wrong with your code

forEach doesnt return anything, it just iterates over the items performing some task
even if forEach did return something you're not returning the result from your method!
el is the element itself, not its index
returning true from inside the method passed to forEach does not return from the eql method itself. Your method will always return false.
... but even if that worked, returning true on the first match will not tell you the entire array equals

Your code can be shortened to

function eql(arr1, arr2) {
    return arr1.length == arr2.length && arr1.every( (x,i) => x == arr2[i]);
}

console.log(eql([],[]));
console.log(eql([1,2,3],[1,2,3]));
console.log(eql([1,2,3],[1,2,2]));
console.log(eql([1,2,3],[1,2,3,4]));

edited Sep 29, 2020 at 7:40

answered Sep 29, 2020 at 7:37

Jamiec

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4 Comments

adiga Over a year ago

eql([,,3],[1,2,3]) returns true because every skips holes.

Jamiec Over a year ago

@adiga interesting, didnt know that. I suspect not so much of a problem in day to day use, but could be a gotcha somewhere down the road.

Barthy Over a year ago

OP asks for a solution with forEach and then states that "this criteria is key". I agree with your solution, but it doesn't seem to fully fit the question…

Jamiec Over a year ago

@Barthy that line was added after this answer was posted. I'll leave it here for future visitors who might not have that requirement

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