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Showing content from https://rustwasm.github.io/docs/wasm-bindgen/reference/attributes/on-js-imports/variadic.html below:

variadic - The `wasm-bindgen` Guide

1. Introduction
3. 1. Examples
4. 1. 1.1. Hello, World!
   2. 1.2. Using console.log
   3. 1.3. Small Wasm files
   4. 1.4. Without a Bundler
   5. 1.5. Synchronous Instantiation
   6. 1.6. Importing functions from JS
   7. 1.7. Working with char
   8. 1.8. js-sys: WebAssembly in WebAssembly
   9. 1.9. web-sys: DOM hello world
   10. 1.10. web-sys: Closures
   11. 1.11. web-sys: performance.now
   12. 1.12. web-sys: using fetch
   13. 1.13. web-sys: Weather report
   14. 1.14. web-sys: canvas hello world
   15. 1.15. web-sys: canvas Julia set
   16. 1.16. web-sys: WebAudio
   17. 1.17. web-sys: WebGL
   18. 1.18. web-sys: WebSockets
   19. 1.19. web-sys: WebRTC DataChannel
   20. 1.20. web-sys: requestAnimationFrame
   21. 1.21. web-sys: A Simple Paint Program
   22. 1.22. web-sys: Wasm in Web Worker
   23. 1.23. Parallel Raytracing
   24. 1.24. Wasm Audio Worklet
   25. 1.25. web-sys: A TODO MVC App
5. 2. Reference
6. 1. 2.1. Deployment
   2. 2.2. JS snippets
   3. 2.3. Static JS Objects
   4. 2.4. Passing Rust Closures to JS
   5. 2.5. Receiving JS Closures in Rust
   6. 2.6. Promises and Futures
   7. 2.7. Iterating over JS Values
   8. 2.8. Arbitrary Data with Serde
   9. 2.9. Accessing Properties of Untyped JS Values
   10. 2.10. Working with Duck-Typed Interfaces
   11. 2.11. Command Line Interface
   12. 2.12. Optimizing for Size
   13. 2.13. Supported Rust Targets
   14. 2.14. Supported Browsers
   15. 2.15. Support for Weak References
   16. 2.16. Support for Reference Types
   17. 2.17. Supported Types
   18. 1. 2.17.1. Imported JavaScript Types
       2. 2.17.2. Exported Rust Types
       3. 2.17.3. JsValue
       4. 2.17.4. Box<[T]> and Vec
       5. 2.17.5. *const T and *mut T
       6. 2.17.6. NonNull
       7. 2.17.7. Numbers
       8. 2.17.8. bool
       9. 2.17.9. char
       10. 2.17.10. str
       11. 2.17.11. String
       12. 2.17.12. Number Slices
       13. 2.17.13. Boxed Number Slices
       14. 2.17.14. Result<T, E>
   19. 2.18. #[wasm_bindgen] Attributes
   20. 1. 2.18.1. On JavaScript Imports
       2. 1. 2.18.1.1. catch
          2. 2.18.1.2. constructor
          3. 2.18.1.3. extends
          4. 2.18.1.4. getter and setter
          5. 2.18.1.5. final
          6. 2.18.1.6. indexing_getter, indexing_setter, and indexing_deleter
          7. 2.18.1.7. js_class = "Blah"
          8. 2.18.1.8. js_name
          9. 2.18.1.9. js_namespace
          10. 2.18.1.10. method
          11. 2.18.1.11. module = "blah"
          12. 2.18.1.12. raw_module = "blah"
          13. 2.18.1.13. no_deref
          14. 2.18.1.14. static_method_of = Blah
          15. 2.18.1.15. structural
          16. 2.18.1.16. typescript_type
          17. 2.18.1.17. variadic
          18. 2.18.1.18. vendor_prefix
       3. 2.18.2. On Rust Exports
       4. 1. 2.18.2.1. constructor
          2. 2.18.2.2. js_name = Blah
          3. 2.18.2.3. js_class = Blah
          4. 2.18.2.4. readonly
          5. 2.18.2.5. skip
          6. 2.18.2.6. skip_jsdoc
          7. 2.18.2.7. start
          8. 2.18.2.8. main
          9. 2.18.2.9. typescript_custom_section
          10. 2.18.2.10. getter and setter
          11. 2.18.2.11. inspectable
          12. 2.18.2.12. skip_typescript
          13. 2.18.2.13. getter_with_clone
          14. 2.18.2.14. unchecked_return_type and unchecked_param_type
          15. 2.18.2.15. return_description and param_description
7. 3. web-sys
8. 1. 3.1. Using web-sys
   2. 3.2. Cargo Features
   3. 3.3. Function Overloads
   4. 3.4. Type Translations
   5. 3.5. Inheritance
   6. 3.6. Unstable APIs
9. 4. Testing with wasm-bindgen-test
10. 1. 4.1. Usage
    2. 4.2. Writing Asynchronous Tests
    3. 4.3. Testing in Headless Browsers
    4. 4.4. Continuous Integration
    5. 4.5. Coverage (Experimental)
11. 5. Contributing to wasm-bindgen
12. 1. 5.1. Testing
    2. 5.2. Internal Design
    3. 1. 5.2.1. JS Objects in Rust
       2. 5.2.2. Exporting a function to JS
       3. 5.2.3. Exporting a struct to JS
       4. 5.2.4. Importing a function from JS
       5. 5.2.5. Importing a class from JS
       6. 5.2.6. Rust Type conversions
       7. 5.2.7. Types in wasm-bindgen
    4. 5.3. js-sys
    5. 1. 5.3.1. Testing
       2. 5.3.2. Adding More APIs
    6. 5.4. web-sys
    7. 1. 5.4.1. Overview
       2. 5.4.2. Testing
       3. 5.4.3. Logging
       4. 5.4.4. Supporting More Web APIs
    8. 5.5. Publishing
    9. 5.6. Team

In javascript, both the types of function arguments, and the number of function arguments are dynamic. For example

function sum(...rest) {
    let i;
    // the old way
    let old_way = 0;
    for (i=0; i<arguments.length; i++) {
        old_way += arguments[i];
    }
    // the new way
    let new_way = 0;
    for (i=0; i<rest.length; i++) {
        new_way += rest[i];
    }
    // both give the same answer
    assert(old_way === new_way);
    return new_way;
}

This function doesn't translate directly into rust, since we don't currently support variadic arguments on the Wasm target. To bind to it, we use a slice as the last argument, and annotate the function as variadic:

# #![allow(unused_variables)]
#fn main() {
#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(variadic)]
    fn sum(args: &[i32]) -> i32;
}
#}

when we call this function, the last argument will be expanded as the javascript expects.

To export a rust function to javascript with a variadic argument, we will use the same bindgen variadic attribute and assume that the last argument will be the variadic array. For example the following rust function:

# #![allow(unused_variables)]
#fn main() {
#[wasm_bindgen(variadic)]
pub  fn  variadic_function(arr: &JsValue) -> JsValue {
    arr.into()
}
#}

will generate the following TS interface

export  function  variadic_function(...arr:  any):  any;

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