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Showing content from https://rustwasm.github.io/docs/wasm-bindgen/reference/types/result.html below:

Result<T, E> - The `wasm-bindgen` Guide

  1. Introduction
  3. 1. Examples
    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
    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
      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
      1. 2.18.1. On JavaScript Imports
        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
        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
    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
    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
    1. 5.1. Testing
    2. 5.2. Internal Design
      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
      1. 5.3.1. Testing
      2. 5.3.2. Adding More APIs
    6. 5.4. web-sys
      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
This documentation is no longer maintained at this domain, and is now maintained at wasm-bindgen.github.io instead. Result<T, E> T parameter &T parameter &mut T parameter T return value Option<T> parameter Option<T> return value JavaScript representation No No No Yes No No Same as T, or an exception

The Result type can be returned from functions exported to JS as well as closures in Rust. The Ok type must be able to be converted to JS, and the Err type must implement Into<JsValue>. Whenever Ok(val) is encountered it's converted to JS and handed off, and whenever Err(error) is encountered an exception is thrown in JS with error.

You can use Result to enable handling of JS exceptions with ? in Rust, naturally propagating it upwards to the Wasm boundary. Furthermore you can also return custom types in Rust so long as they're all convertible to JsValue.

Note that if you import a JS function with Result you need #[wasm_bindgen(catch)] to be annotated on the import (unlike exported functions, which require no extra annotation). This may not be necessary in the future though and it may work "as is"!.

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