tsup version: 6.6.3

1. Write a module without export

// center.ts
declare global {
    interface String {
        center(width: number, fillChar?: string): string; 
    }
}

Sring.prototype.center=function (width:number,fillChar=" ") {
    let s=this
    let llength=Math.floor((width-s.length)/2)
    return new Array(llength).join(fillChar)+s+new Array(width-s.length-llength).join(fillChar)
}
export {}               // necessary

2. tsup.config.js

import { defineConfig } from 'tsup'

export default defineConfig({
    entry: [
        'src/*.ts'
    ],
    format: ['esm','cjs'],
    dts: true,
    splitting: true,
    sourcemap: false,
    clean: true,
    treeshake:true,  
    minify: true,

3. build result

// center.d.ts

declare global {
    interface String {
        center(width: number, fillChar?: string): string; 
    }
}

export {} removed,causes String.prototype.center to be unrecognized。

I have to use a hacker method:

// center.ts
declare global {
    interface String {
        center(width: number, fillChar?: string): string; 
    }
}

Sring.prototype.center=function (width:number,fillChar=" ") {
    let s=this
    let llength=Math.floor((width-s.length)/2)
    return new Array(llength).join(fillChar)+s+new Array(width-s.length-llength).join(fillChar)
}
export const _=0              // replace for export {}