Why does the compiler think `v: string | null` might be null after `v && v.trim()`?

[short holly]

bawdyinkslinger#0

Playground Link

Preview:```ts
declare const payload: {
contents: string
name: string
}[]
let content = ""
let onOpen: string | null = null
let onClose: string | null = null

payload.forEach(function (payload, index) {
if (index === 0) {
content = payload.contents
} else {
if (payload.name === "on
...```

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[ornate swift]

Why do I get this error message? Shouldn't it be impossible for onOpen to be null there?

Neither of those types make sense to me

[green flame]

Ah, this is a classic issue.

[ornate swift]

🙄 Why do I keep running into those?

[green flame]

You need to do let onOpen = null as (string | null);

This is a place where the contextual flow typing gets confused.

[ornate swift]

What's happening here?

[short holly]

bawdyinkslinger#0

Playground Link

Preview:```ts
declare const payload: {
contents: string
name: string
}[]
declare const takeString: (s: string) => void
let content = ""
let onOpen = null as string | null
let onClose = null as string | null

payload.forEach(function (payload, index) {
if (index === 0) {
content = payload.conte
...```

You can choose specific lines to embed by selecting them before copying the link.

[ornate swift]

do I need an as string to solve this one?

This is the real code that creates that error: ```ts
$(document).one(':dialogopened', function () {
$.wiki(onOpen);
});

[green flame]

That code doesn't really make much sense with the extra function wrapping?

[ornate swift]

I can read minds but I can't figure out TS's type system 🙂

Argument of type 'string | null' is not assignable to parameter of type 'string'.
Type 'null' is not assignable to type 'string'.

[green flame]

This one is more reasonable. TS doesn't narrow (mutable things) across function boundaries.

You can 'save' the narrowed value into a constant.

[ornate swift]

Oh I don't know how I never noticed that

I guess I don't really write code like this often. I'm trying to port some code I found in a js library

[green flame]

Yeah, I think most people use a lot of const which avoids the issue

[ornate swift]

yeah, I suppose I normally iterate that payload three times to get the three variables out of it, or maybe have a const { onOpen, ...} as the left-hand side

Would you do something vastly different?

[green flame]

If you can it may be easier to just move the check inside the event and avoid the issue

Not 100% the same, but I'd guess in most cases an event handler that does nothing is basically the same as not setting the event handler at all.