Live, in-editor highlighting of crash-prone JavaScript patterns — the bugs that throw TypeError: Cannot read properties of undefined, hang requests on un-settled promises/callbacks, or send duplicate HTTP responses.

It squiggles only the lines you have changed vs the last git commit (HEAD), so it focuses on the code you're writing — not the whole legacy file.

What it flags

Rule	Pattern	Example
`#1`	Unguarded null/undefined access	`req.body.user.email.trim()`
`#2`	Array index access	`arr[0].name`
`#22`	Missing optional chaining	`a.b.c.d`
`#24`	String method on possibly-undefined	`value.trim()`
`#25`	Array method on possibly-undefined	`users.map(...)`
`#35`	`for…of` over a nullable source	`for (const x of arr)`
`#19`	`JSON.parse` without try/catch	`JSON.parse(data)`
`#26`	Destructuring `null`/`undefined`	`const { x } = null`
`#10`	`forEach` with an async callback	`items.forEach(async i => { await … })`
`#7`	Promise that never settles	`new Promise((resolve) => { /* never calls resolve */ })`
`#11`	`await` without try/catch	`const u = await getUser();`
`#12`	Missing `return` after a response	`res.json(x); next();`
`#5`	`async` library task that never calls its callback (incl. only-on-some-paths)	`async.parallel({ a: fn(next){ if(x) next() } })`
`#9`	Forgotten `return` in `.map`/`.filter`/`.reduce`/…	`arr.map(x => { x.name; })`
`#16`	Assignment used as a condition	`if (status = 1)`
`#17`	Loose equality	`a == b`, `a != b`
`#20`	`switch` without a `default` case	`switch(r){ case 1: }`
`#21`	Infinite loop (constant / unmodified condition)	`while(c < 10){ /* c never changes */ }`
`#29`	Empty catch / empty block	`catch (e) {}`

Each diagnostic shows what's wrong and how to fix it on hover.

What it deliberately does NOT flag (to avoid noise)

Optional-chained or guarded access: a?.b?.c, (arr || []).map(), if (x) x.trim()
Known-safe receivers: req.body, req.session, res.json(), e.target, formik.values
require() bindings and builder chains: Model.findOne().lean(), arr.filter().map()
Terminal converters: id.toString(), doc.toObject()

Settings

Setting	Default	Description
`runtimeSafety.enable`	`true`	Turn diagnostics on/off
`runtimeSafety.severity`	`warning`	`error` \| `warning` \| `information` \| `hint`
`runtimeSafety.rules.nullSafety`	`true`	Null/undefined access rules
`runtimeSafety.rules.patterns`	`true`	Promise/callback/control-flow patterns