I hear this all the time from JS fans. It’s rather like BS. First of all, JS is primarily an imperative/procedural language with some functional and object-oriented capabilities tacked on. Just like C++ and Python, you can write JS code without using any FP or OOP at all.
Second, JS is functional because it has first-class functions and closures. This is not at all special, as most other languages today have first-class functions and closures (such as C#, C++, PHP, Perl, Ruby, Go, Smalltalk, etc.). We might as well call all programming languages “functional.”
Third, JS does not have immutability as a language feature. Most FP purists would insist on immutability. (The fact that an immutability library is available for JS doesn’t make it a true FP language. Similar libraries can be written for any language.)