Array objects error out when setting a property

[tiagovtristao]

Bug Report

Array objects error out when setting a property.

🔎 Search Terms

"array object"

🕗 Version & Regression Information

I've tested it in both v3 and v4.

⏯ Playground Link

https://www.typescriptlang.org/play?#code/DYUwLgBAZg9jEF4IG0C6BuAUJ2MB0AtgJ4AKATjAA6IQCM6QA

💻 Code

let foo = [];

foo.myProp = 1;

🙁 Actual behavior

"Property 'myProp' does not exist on type 'any[]'."

🙂 Expected behavior

It shouldn't error out.

[MartinJohns]

You probably need to expand on that. Why do you think it shouldn't error, when the type definition declares no such property?

[tiagovtristao]

I can't easily get that to work without creating some elaborate TypeScript types to make it valid. I feel that shouldn't be the case.

The following is similar for function objects as it works fine:

function foobar() {}

foobar.myProp = 1;

[jcalz]

This is intentional behavior and not a bug in TypeScript. Perhaps you want a feature request to treat all objects (or maybe just arrays for some reason) as expando objects, the way function statements are currently. I wonder if this request has happened before...

ah, looks like #12416

[MartinJohns]

I can't easily get that to work without creating some elaborate TypeScript types to make it valid.

Not really that elaborate:

type Foo = Array<string> & { myProp?: number };
let foo: Foo = [];
foo.myProp = 1;

[tiagovtristao]

I can't easily get that to work without creating some elaborate TypeScript types to make it valid.

Not really that elaborate:

type Foo = Array<string> & { myProp?: number };
let foo: Foo = [];
foo.myProp = 1;

That works if you want the properties to be optional.

[tiagovtristao]

@RyanCavanaugh, do you know if there's any chance of this being implemented, if I raise a feature request?

I came across this issue when issue https://styled-system.com/responsive-styles#using-objects.

[RyanCavanaugh]

Unlikely. This would be a rather huge inference revamp for an initialization style that doesn't see much use.

[tiagovtristao]

Fair enough, thanks.