how to use morse code dictionary provided by author in JavaScript?

You should use the Morse code they gave you:

The Morse code table is preloaded for you as a dictionary, feel free to use it:

Coffeescript/C++/Go/JavaScript/Julia/PHP/Python/Ruby/TypeScript: MORSE_CODE['.--']
C#: MorseCode.Get(".--") (returns string)
F#: MorseCode.get ".--" (returns string)
Elixir: @morse_codes variable (from use MorseCode.Constants). Ignore the unused variable warning for morse_codes because it's no longer used and kept only for old solutions.
Elm: MorseCodes.get : Dict String String
Haskell: morseCodes ! ".--" (Codes are in a Map String String)
Java: MorseCode.get(".--")
Kotlin: MorseCode[".--"] ?: "" or MorseCode.getOrDefault(".--", "")
Racket: morse-code (a hash table)
Rust: MORSE_CODE
Scala: morseCodes(".--")
Swift: MorseCode[".--"] ?? "" or MorseCode[".--", default: ""]
C: provides parallel arrays, i.e. morse[2] == "-.-" for ascii[2] == "C"
NASM: a table of pointers to the morsecodes, and a corresponding list of ascii symbols
All the test strings would contain valid Morse code, so you may skip checking for errors and exceptions. In C#, tests will fail if the solution code throws an exception, please keep that in mind. This is mostly because otherwise the engine would simply ignore the tests, resulting in a "valid" solution.

In addition to letters, digits and some punctuation, there are some special service codes, the most notorious of those is the international distress signal SOS (that was first issued by Titanic), that is coded as ···−−−···. These special codes are treated as single special characters, and usually are transmitted as separate words.

Probably related to that?