Live-highlights crash-prone JavaScript patterns on the lines you change: unguarded null/undefined access, hanging promises/callbacks, forEach-async, unsafe iteration and more.
Installation
Launch VS Code Quick Open (Ctrl+P), paste the following command, and press enter.
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()
