When added to a const &'static str, it will append the contents of the string to the .d.ts file exported by wasm-bindgen-cli (when the --typescript flag is enabled).

#![allow(unused_variables)]
fn main() {
#[wasm_bindgen(typescript_custom_section)]
const TS_APPEND_CONTENT: &'static str = r#"

export type Coords = { "latitude": number, "longitude": number, }; 

"#;
}

The primary target for this feature is for code generation. For example, you can author a macro that allows you to export a TypeScript definition alongside the definition of a struct or Rust type.

#![allow(unused_variables)]
fn main() {
#[derive(MyTypescriptExport)]
struct Coords {
    latitude: u32,
    longitude: u32,
}
}

The proc_derive_macro "MyTypescriptExport" can export its own #[wasm_bindgen(typescript_custom_section)] section, which would then be picked up by wasm-bindgen-cli. This would be equivalent to the contents of the TS_APPEND_CONTENT string in the first example.

This feature allows plain data objects to be typechecked in Rust and in TypeScript by outputting a type definition generated at compile time.