JavaScript is also an extremely bad functional language unless you don't mind ridiculous memory bloat and inefficiency. To be even reasonably efficient you're basically forced to use classes because the primary ergonomic alternative is objects that contain functions that close over lexical scope. This is ridiculously inefficient because functions that would normally have a single instance on the prototype chain are now duplicated for every single object. Modern JavaScript engines can't optimize this without breaking the spec which they obviously won't do. You can resort to just using plain functions and objects that contain nothing but data but this quickly becomes quite ugly without a pipeline operator.

The standard library is also filled with mutating methods and doesn't include proper functional data structures and pure JavaScript implementations of functional data structures, such as immutable.js, are so slow that you might as well just use arrays and objects and repeatedly do shallow copies.

Programs that stress functional purity in JavaScript are becoming hard to meaningfully benchmark because instead of having a few hotspots absolutely everything is incredibly inefficient and slow. Death by a thousand cuts.