on type safety in LangChain TS

snippet-5.ts

/**
   * Parses the given text and returns an `AgentAction` or `AgentFinish`
   * object. If an `OutputFixingParser` is provided, it is used for parsing;
   * otherwise, the base parser is used.
   * @param text The text to parse.
   * @param callbacks Optional callbacks for asynchronous operations.
   * @returns A Promise that resolves to an `AgentAction` or `AgentFinish` object.
   */
  async parse(text: string): Promise<AgentAction | AgentFinish> {
    try {
      const regex = /```(?:json)?(.*)(```)/gs;
      const actionMatch = regex.exec(text);
      if (actionMatch === null) {
        throw new OutputParserException(
          `Could not parse an action. The agent action must be within a markdown code block, 
          and "action" must be a provided tool or "Final Answer"`
        );
      }
      const response = JSON.parse(actionMatch[1].trim());
      const { action, action_input } = response;

      if (action === "Final Answer") {
        return { returnValues: { output: action_input }, log: text };
      }
      return { tool: action, toolInput: action_input || {}, log: text };
    } catch (e) {
      throw new OutputParserException(
        `Failed to parse. Text: "${text}". Error: ${e}`
      );
    }
  }