Why [] == ![] in JavaScript: Understanding Loose Equality and Its Pitfalls

This article examines the puzzling JavaScript expression [] == ![] and explains why it evaluates to true despite seeming illogical.

What Happens Internally

In JavaScript an empty array [] is a truthy value, so applying the logical NOT operator yields false. During loose equality comparison the engine then coerces both sides to primitive values: [] becomes an empty string "", which is further converted to the number 0, while false also becomes 0. Since 0 == 0, the original expression evaluates to true.

Because the empty array is truthy, ![] is false. The comparison then proceeds as [] == false, which follows the same coercion steps described above.

Why [] == [] Is False

When two arrays are compared with == (or ===) JavaScript does not compare their contents; it compares references. Two distinct array objects never share the same reference, so the result is always false.

How to Compare Arrays Properly

If the arrays are sorted, you can compare their JSON string representations:

JSON.stringify(arr1) === JSON.stringify(arr2)

For unsorted arrays a more generic approach is to compare lengths and then check each element with every():

arr1.length === arr2.length && arr1.every((v,i)=> v===arr2[i])

Final Thoughts

The loose equality operator == can produce results that make no sense in real‑world logic, such as [] == ![]. The safest practice is to always use strict equality ===, adopt TypeScript for stronger type checking, and rely on modern language features to avoid these pitfalls.