You have a 26 in your code, where does it come from? Don't hardcode that value.

It's not a kata issue, it's a problem with your code, use Question label next time:

JavaScript Completions 1274

Tried your code in repl.it and it's returning wrong results. If it helps to debug your code, try this in the sample tests:

var abc, key;
abc = "アイウエオァィゥェォカキクケコサシスセソタチツッテトナニヌネノハヒフヘホマミムメモヤャユュヨョラリルレロワヲンー";
key = "カタカナ"
c = new VigenèreCipher(key, abc);

Test.assertEquals(c.encode('カタカナ'), 'タモタワ');
Test.assertEquals(c.decode('タモタワ'), 'カタカナ');

This works

the test wont pass it

Choose one.