The findIndex() method of Array instances returns the index of the first element in an array that satisfies the provided testing function. If no elements satisfy the testing function, -1 is returned.

The toReversed() method of Array instances is the copying counterpart of the reverse() method. It returns a new array with the elements in reversed order.

The reverse() method of Array instances reverses an array in place and returns the reference to the same array, the first array element now becoming the last, and the last array element becoming the first. In other words, elements order in the array will be turned towards the direction opposite to that previously stated.

The at() method of Array instances takes an integer value and returns the item at that index, allowing for positive and negative integers. Negative integers count back from the last item in the array.

The Atomics.sub() static method subtracts a given value at a given position in the array and returns the old value at that position. This atomic operation guarantees that no other write happens until the modified value is written back.

The Atomics.store() static method stores a given value at the given position in the array and returns that value.

The Atomics.isLockFree() static method is used to determine whether the Atomics methods use locks or atomic hardware operations when applied to typed arrays with the given element byte size. It is intended as an optimization primitive, so that high-performance algorithms can determine whether to use locks or atomic operations in critical sections. If an atomic primitive is not lock-free, it is often more efficient for an algorithm to provide its own locking.

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The has() method of Set instances returns a boolean indicating whether an element with the specified value exists in this set or not.