langchain.js
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    Namespace Annotation

    Helper that instantiates channels within a StateGraph state.

    Can be used as a field in an Annotation.Root wrapper in one of two ways:

    1. Directly: Creates a channel that stores the most recent value returned from a node.
    2. With a reducer: Creates a channel that applies the reducer on a node's return value.
    import { StateGraph, Annotation } from "@langchain/langgraph";

    // Define a state with a single string key named "currentOutput"
    const SimpleAnnotation = Annotation.Root({
      currentOutput: Annotation<string>,
    });

    const graphBuilder = new StateGraph(SimpleAnnotation);

    // A node in the graph that returns an object with a "currentOutput" key
    // replaces the value in the state. You can get the state type as shown below:
    const myNode = (state: typeof SimpleAnnotation.State) => {
      return {
        currentOutput: "some_new_value",
      };
    }

    const graph = graphBuilder
      .addNode("myNode", myNode)
      ...
      .compile();

    import { type BaseMessage, AIMessage } from "@langchain/core/messages";
    import { StateGraph, Annotation } from "@langchain/langgraph";

    // Define a state with a single key named "messages" that will
    // combine a returned BaseMessage or arrays of BaseMessages
    const AnnotationWithReducer = Annotation.Root({
      messages: Annotation<BaseMessage[]>({
        // Different types are allowed for updates
        reducer: (left: BaseMessage[], right: BaseMessage | BaseMessage[]) => {
          if (Array.isArray(right)) {
            return left.concat(right);
          }
          return left.concat([right]);
        },
        default: () => [],
      }),
    });

    const graphBuilder = new StateGraph(AnnotationWithReducer);

    // A node in the graph that returns an object with a "messages" key
    // will update the state by combining the existing value with the returned one.
    const myNode = (state: typeof AnnotationWithReducer.State) => {
      return {
        messages: [new AIMessage("Some new response")],
      };
    };

    const graph = graphBuilder
      .addNode("myNode", myNode)
      ...
      .compile();

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