Calling JavaScript

h5 provides several mechanisms to interact with existing JavaScript code and libraries.

Using Script.Write and Script.Eval

The most direct way to call JavaScript is by using Script.Write or Script.Eval from the H5 namespace.

Script.Write

Script.Write injects raw JavaScript code directly into the compiled output.

using H5;

public void SayHello()
{
    // Emits: alert("Hello from JavaScript!");
    Script.Write("alert('Hello from JavaScript!');");
}

You can also use placeholders for arguments:

public void LogMessage(string message)
{
    // Emits: console.log(message);
    Script.Write("console.log({0});", message);
}

Script.Eval

Script.Eval evaluates a JavaScript expression at runtime and returns the result.

public int Add(int a, int b)
{
    // Evaluates: a + b
    return Script.Eval<int>($"{a} + {b}");
}

Using Script.Call, Script.Get, Script.Set

Helper methods are available to interact with JavaScript objects and functions dynamically.

using H5;

// Call a global function
Script.Call("myFunction", "arg1", 123);

// Get a global variable
var title = Script.Get<string>("document.title");

// Set a global variable
Script.Set("window.myVar", "value");

Integrating External Code

For a more type-safe approach, you can define external interfaces using the [External] attribute.

[External] Attribute

Use [External] to define C# bindings for existing JavaScript libraries. The compiler will not generate implementation code for these types but will assume they exist at runtime.

[External]
[Name("Math")]
public static class Math
{
    [Name("random")]
    public static extern double Random();

    [Name("max")]
    public static extern double Max(double a, double b);
}

// Usage
double r = Math.Random();

[Template] Attribute

The [Template] attribute allows you to specify exactly how a method call should be translated to JavaScript. This is powerful for adapting C# method signatures to JavaScript APIs.

[External]
public static class Console
{
    [Template("console.log({0})")]
    public static extern void Log(object message);

    [Template("console.warn({0}, {1})")]
    public static extern void Warn(string category, string message);
}

When you call Console.Log("Hello"), h5 generates console.log("Hello").

[Script] Attribute

The [Script] attribute allows you to inject a script block directly into the generated code for a method.

public class MyClass
{
    [Script("alert('Hello from inline script!');")]
    public void InlineScript()
    {
    }
}

This is similar to Script.Write but declarative.