feat(js): typed context

[Ehesp]

Currently when dealing with Context in Genkit, the type returned everywhere is a static ActionContext, which looks like:

interface ActionContext {
    /** Information about the currently authenticated user if provided. */
    auth?: Record<string, any>;
    [additionalContext: string]: any;
}

When providing context within Genkit, e.g. within flows, you lose all type-safety:

const flow = ai.defineFlow(..., 
  async (input, { context }) => {
    context.hello('world');
  }
);

flow(..., {
  context: {
    hello: (value: string) => console.log(value),
  }
});

Here, context.hello is an any type. This means you end up having to write your own definitions, and casting context:

const flow = ai.defineFlow(..., 
  async (input, { context }) => {
    const ctx = context as unknown as { hello: (value: string) => void };
    ctx.hello('world');
  }
);

This then doesn't scale, as context is inherited everywhere, and it becomes a bit PITA to type, resulting in bad DX.

This PR enables typed and inferred context typing:

Inferred:

const ai = genkit({
  context: {
    hello: (name: string) => `Hello, ${name}!`,
  },
});

ai.defineFlow(..., 
  // context.hello is typed!
  async (input, { context }) => context.hello(input)
);

Declared:

type MyContext = {
  hello: (name: string) => void;
}

const ai = genkit<MyContext>({
  context: {
    hello: (name) => `Hello, ${name}!`,
  },
});

ai.defineFlow(..., 
  // context = MyContext & ActionContext
  async (input, { context }) => context.hello(input)
);

Additionally, context returned from actions is currently typed as ActionContext | undefined, which caused more issues having to do context?.something. However looking at the code, context is always defined, even if the result is an empty object:

              context: {
                ...actionFn.__registry?.context,
                ...(options?.context ?? getContext()),
              },

So this PR also removes the | undefined type (happy to be proved wrong here!). Note, I don't think this is a breaking change.

Because this PR allows typed context, I'd of ideally liked to remove the [additionalContext: string]: any; within ActionContext:

interface ActionContext {
    /** Information about the currently authenticated user if provided. */
    auth?: Record<string, any>;
-    [additionalContext: string]: any;
}

However, technically I think this would end up being a breaking change since users relying on the any would all of a sudden get type errors.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the developer experience in Genkit by introducing robust type safety for application-specific context within flows and tools. By allowing developers to define and infer custom context types, it eliminates the need for manual type casting and reduces potential runtime errors, making Genkit applications more predictable and easier to maintain. The changes also clarify that context is always available, streamlining its usage.

Highlights

• Typed Context Implementation: Introduced a generic type parameter 'C' across Genkit's core interfaces and classes (e.g., Genkit, ActionFnArg, FlowFn, ToolFn) to enable strong typing for custom application context within flows and tools.
• Improved Developer Experience: Updated context parameters in flow and tool definitions to automatically infer or accept declared custom types, eliminating the need for manual type casting and enhancing type safety and predictability.
• Context Always Defined: Refined the 'context' property within 'ActionFnArg' to be non-optional ('context: C & ActionContext'), clarifying that context is always provided during action execution, even if empty.
• New Test Application: Added a new test application ('js/testapps/typed-context') to provide a practical demonstration and validation of the new typed context functionality.

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Changelog

• js/ai/src/tool.ts
  • Removed direct import of 'ActionContext' as it's now handled via generic types.
  • Updated 'ToolFnOptions' to accept a generic context type 'C' and extend 'ActionFnArg<never, C>'.
  • Modified type signatures for 'ToolFn', 'MultipartToolFn', 'defineTool', 'tool', 'basicTool', 'basicToolV2', 'multipartTool', and 'dynamicTool' to incorporate the generic context type 'C'.
  • Added type assertions for context parameters in tool execution functions.
• js/core/src/action.ts
  • Updated 'ActionFnArg' interface to accept a generic context type 'C'.
  • Changed the 'context' property within 'ActionFnArg' from optional ('context?: ActionContext') to required ('context: C & ActionContext'), merging custom context with the base 'ActionContext'.
• js/core/src/context.ts
  • Corrected a typo in the JSDoc comment for 'ActionContext'.
• js/core/src/flow.ts
  • Imported 'ActionContext' for use in type definitions.
  • Updated 'FlowSideChannel' interface to accept a generic context type 'C' and extend 'ActionFnArg<S, C>'.
  • Modified type signatures for 'FlowFn', 'flow', 'defineFlow', and 'flowAction' to incorporate the generic context type 'C'.
  • Added type assertions for 'FlowSideChannel' context assignments.
• js/genkit/src/genkit.ts
  • Updated 'GenkitOptions' interface to accept a generic context type 'C'.
  • Made the 'Genkit' class generic over 'C'.
  • Modified the constructor, 'defineFlow', 'defineTool', 'dynamicTool', 'defineModel', 'generate', 'generateStream', and 'currentContext' methods to use and propagate the generic context type 'C'.
  • Added JSDoc comments to explain the new generic context type parameter for better documentation.
• js/pnpm-lock.yaml
  • Added new dependency entries for the 'testapps/typed-context' project.
• js/testapps/typed-context/README.md
  • Added a new README file describing the typed context test application, its features, setup, and testing instructions.
• js/testapps/typed-context/package.json
  • Added a new 'package.json' file for the 'typed-context' test application, including scripts and dependencies.
• js/testapps/typed-context/src/index.ts
  • Added a new TypeScript source file demonstrating the definition and usage of typed context with 'genkit()', flows, and tools.
• js/testapps/typed-context/tsconfig.json
  • Added a new TypeScript configuration file for the 'typed-context' test application.

Activity

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[gemini-code-assist]

Code Review

This pull request is a great improvement for Genkit's developer experience, introducing type-safety for context objects in flows and tools. The changes are well-implemented across the core, AI, and Genkit packages, enabling both inferred and declared context typing. The removal of | undefined from the context type is also a welcome change that simplifies usage. I've found a couple of minor issues: one is a redundant type cast that can be removed, and the other is an incorrect README file for the new test application. Overall, this is a solid feature addition.