I wouldn't use Java or C# for that comparison, either (though yes, it's a safer bet that Java will be popular 20 years from now -- Javascript has only become popular as a language in the last 5-10 years, whereas Java has been popular since the 90s.)

But would I bet on Javascript over (for example) C? No way. Any new developer would be wise to learn C, even if they completely ignored Javascript. And I don't need any technical details about the language to make that call.