private

The private attribute can be applied to exported structs and enums to generate the exported binding but not export it on the public exports of the JavaScript module. This allows defining types that can be used as arguments or return values in exported functions without exposing them in the public exported API.

#[wasm_bindgen(private)]
pub struct Config {
    pub timeout: i32,
}

#[wasm_bindgen]
pub fn create_config(timeout: i32) -> Config {
    Config { timeout }
}

#[wasm_bindgen]
pub fn apply_config(config: Config) -> i32 {
    config.timeout * 2
}

This will generate JavaScript where Config is defined internally but not exported:

import * as app from './app';

// app.Config is undefined - not exported
console.log(app.Config); // undefined

// But the functions that use Config are exported
const config = app.create_config(100);
const result = app.apply_config(config);

The TypeScript definitions will still export the type under its name as a type-only export:

export function create_config(timeout: number): Config;
export function apply_config(config: Config): number;

class Config {
    timeout: number;
}

export type { Config };

The private attribute is only supported on structs and enums, not on functions.

Name collisions

Because private types are not part of the public JavaScript API, multiple crates in the same build may each export a private type with the same name without colliding. This is useful for crates that use private types as implementation details: two versions of such a crate can coexist in one dependency graph. When the same name is defined more than once, the private copies are numbered (Config, Config2, Config3, ...) in the generated bindings.

Exporting the same name publicly from more than one crate remains an error, reported by the wasm-bindgen CLI with the qualified name (taking js_namespace into account).