.reg script:

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\.js]
@="JSFile"

manually: use regedit:
go to HKEY_CLASSES_ROOT\.js:
set (Default) = JSFile

assoc .js=JSFILE did not work for me: assoc was not editing the registry, and cscript.exe was reading from the registry:
whatever value is set as (Default), cscript.exe will attempt to read the following key: HKEY_CLASSES_ROOT\%s\ScriptEngine, normally, it would be HKEY_CLASSES_ROOT\JSFile\ScriptEngine

I got this error while running cscript.exe on a script.js

cscript.exe //NoLogo AutoHotkey_alpha\source\scripts\minman.js