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Imports

In WebAssembly terms, Imports are the external functions, tables, memories and globals a Wasm module or component requires. In order to use an instance of the specific Wasm module or component, it is necessary to pass the various entities listed under this tab to an instance of the module or component.

Examples

Passing imports into a Wasm module from might look something like this:

JavaScript
Rust, with Wasmtime

// Creating an object with references to the imports specified in the WebAssembly file.
const importsObject = {
  imports: {
    imported_function: aFunction,
    imported_table: aTable,
    imported_memory: aMemory,
    imported_global: aGlobal,
  },
};

// Fetching and instantiating a module or object from the WebAssembly file.
WebAssembly.instantiateStreaming(
  fetch("moduleOrComponent.wasm"),
  importsObject
).then((result) => {
  // Process result
});

fn main() -> Result<(), Box<dyn Error>> {
    let engine = Engine::default();
    let mut store = Store::new(&engine, ());
    let module = Module::from_file(&engine, "moduleOrComponent.wasm")?;

    // We'll use a linker for linking functions and globals to instance.
    let mut linker = Linker::new(&engine);

    linker.func_wrap("", "imported_function", a_function)?;
    linker.define(&store, "", "imported_global", a_global)?;

    // Passing memory and global into instance constructor
    let instance = Instance::new(&mut store, &module, &[memory.into(), table.into()])?;

    // Linker will match named imports to existing linked functions and globals.
    linker.instance(&mut store, "anInstance", instance)?;

    // code to use instance goes here

    Ok(())
}

Additional reading

For further information about the Imports concept, have a look at Mozilla’s documentation on the subject.

You can also read about how Wasm imports are displayed in the Nor² Wasm Inspector in the Imports page section of our WASM Analyzer documentation.