Why Doesn't `==` Work for Comparing Arrays in JavaScript?

Why Equality Check Fails for Arrays

In JavaScript, equality check (==) between arrays returns false, even when the arrays appear to contain identical elements. This behavior contradicts the intuitive expectation of value equality.

Understanding the Nature of Arrays

JavaScript arrays are not primitive data types but objects. When comparing two arrays using ==, the operator checks if both arrays are the exact same instance, not if their contents are equal.

How to Compare Array Contents

To determine if two arrays have the same contents, you need to explicitly compare each corresponding element. Here's a simple function to do this:

function arraysEqual(arr1, arr2) {
  if (arr1.length != arr2.length) {
    return false;
  }
  for (let i = 0; i < arr1.length; i++) {
    if (arr1[i] != arr2[i]) {
      return false;
    }
  }
  return true;
}

JSON.stringify() Fallacy

Some suggest using JSON.stringify() to convert both arrays into JSON strings and then compare the strings. While this may work in certain scenarios, it's not a reliable solution. JSON.stringify() maintains object property order in the resulting string, which may vary depending on implementation details. This inconsistency can lead to false negatives in equality checks.

Best Practice

For reliable comparison of array contents, it's recommended to write a custom function that iterates over all elements and explicitly checks for equality. This approach ensures accuracy and robustness in comparing arrays.

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