prompt_template.ts

import { Document } from "@langchain/core/documents";
import { BasePromptTemplate } from "@langchain/core/prompts";

/**
 * Formats a document using a given prompt template.
 *
 * @async
 * @param {Document} document - The document to format.
 * @param {BasePromptTemplate} prompt - The prompt template to use for formatting.
 * @returns {Promise<string>} A Promise that resolves to the formatted document as a string.
 * @throws {Error} If the document is missing required metadata variables specified in the prompt template.
 */
export const formatDocument = async (
  document: Document,
  prompt: BasePromptTemplate
): Promise<string> => {
  const baseInfo = {
    pageContent: document.pageContent,
    ...document.metadata,
  };
  const variables = new Set(prompt.inputVariables);
  const requiredMetadata = new Set(
    prompt.inputVariables
      .map((v) => (v !== "pageContent" ? v : null))
      .filter((v) => v !== null)
  );
  const missingMetadata = [];
  for (const variable of variables) {
    if (!(variable in baseInfo) && variable !== "pageContent") {
      missingMetadata.push(variable);
    }
  }
  if (missingMetadata.length) {
    throw new Error(
      `Document prompt requires documents to have metadata variables: ${JSON.stringify(
        requiredMetadata
      )}. Received document with missing metadata: ${JSON.stringify(
        missingMetadata
      )}`
    );
  }
  return prompt.format(baseInfo);
};