[JS] Straightforward one-liner
\nconst countConsistentStrings = (allowed, words) => \n    words.filter(\n        word => [...word].every(letter => allowed.includes(letter))\n    ).length\n
```\nconst countConsistentStrings = (allowed, words) => \n    words.filter(\n        word => [...word].every(letter => allowed.includes(letter))\n    ).length\n```
['JavaScript']