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.