Adding custom method to Object.prototype in TypeScript

Question

I have the following code:

Object.prototype.custom = function() {
    return this
}

It works just fine in JavaScript, but when I put it in TypeScript, I get this error:

Property 'custom' does not exist on type 'Object'.ts(2339)

How can I bypass or solve this complaint?

Just don't add anything to global prototypes. (Don't do that in JS either!) Please, at least don't add anything to Object.prototype! Especially, don't make it enumerable!!! (Your code will add that property to every single object in existence, not even only the ones you manually create, and will even show up as a key in for..in loops, which is bad. Like, very bad. Doing that in your project is shooting yourself in the foot. Doing that in a library is shooting everyone in the foot.) — FZs
– FZs, Commented Sep 24, 2022 at 5:37

hgb123 · Accepted Answer · 2022-09-24 05:32:39Z

5

For the sake of the experiment (not advised in production, IMO), you could either ignore it or extend Object (aka augmentation)

// @ts-ignore
Object.prototype.custom = function() {
    return this
}

interface Object {
  custom2(): Object;
}

Object.prototype.custom2 = function() {
    return this
}

TS playground

answered Sep 24, 2022 at 5:32

hgb123

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1 Comment

jcalz Over a year ago

Might want to link to docs on declaration merging as well as possibly talking about global augmentation if the code is in a module. But yeah, I agree that this is a BAD IDEA.

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