---
title: Loader Tutorial
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---

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Many existing systems and platforms include support for loading data from CSV
files. Many users prefer to work in spreadsheet software and multi-sheet file
formats including XLSX. SheetJS libraries help bridge the gap by translating
complex workbooks to simple CSV data.

The goal of this example is to load spreadsheet data into a vector store and use
a large language model to generate queries based on English language input. The
existing tooling supports CSV but does not support real spreadsheets.

In ["SheetJS Conversion"](#sheetjs-conversion), we will use SheetJS libraries to
generate CSV files for the LangChain CSV loader. These conversions can be run in
a preprocessing step without disrupting existing CSV workflows.

In ["SheetJS Loader"](#sheetjs-loader), we will use SheetJS libraries in a custom
loader to directly generate documents and metadata.

:::note Tested Deployments

This demo was tested in the following configurations:

| Date       | Platform                                                      |
|:-----------|:--------------------------------------------------------------|
| 2024-06-19 | Apple M2 Max 12-Core CPU + 30-Core GPU (32 GB unified memory) |
| 2024-06-19 | NVIDIA RTX 4080 SUPER (16 GB VRAM) + i9-10910 (128 GB RAM)    |

This explanation was verified against LangChain 0.2.

:::

## CSV Loader

Document loaders generate data objects ("documents") and associated metadata
from data sources.

LangChain offers a `CSVLoader`[^1] component for loading CSV data from a file:

```js title="Generating Documents from a CSV file"
import { CSVLoader } from "@langchain/community/document_loaders/fs/csv";

const loader = new CSVLoader("pres.csv");
const docs = await loader.load();

console.log(docs);
```

The CSV loader uses the first row to determine column headers and generates one
document per data row. For example, the following CSV holds Presidential data:

```csv
Name,Index
Bill Clinton,42
GeorgeW Bush,43
Barack Obama,44
Donald Trump,45
Joseph Biden,46
```

Each data row is translated to a document whose content is a list of attributes
and values. For example, the third data row is shown below:

<table>
  <thead><tr><th>CSV Row</th><th>Document Content</th></tr></thead>
  <tbody><tr><td>

```
Name,Index
Barack Obama,44
```

  </td><td>

```
Name: Barack Obama
Index: 44
```

  </td></tr></tbody>
</table>

The LangChain CSV loader will include source metadata in the document:

```js title="Document generated by the CSV loader"
Document {
  pageContent: 'Name: Barack Obama\nIndex: 44',
  metadata: { source: 'pres.csv', line: 3 }
}
```

## SheetJS Conversion

The [SheetJS NodeJS module](/docs/getting-started/installation/nodejs) can be
imported in NodeJS scripts that use LangChain and other JavaScript libraries.

A simple pre-processing step can convert workbooks to spreadsheets

```mermaid
flowchart LR
  file[(Workbook\nXLSX/XLS)]
  subgraph SheetJS Structures
    wb(((SheetJS\nWorkbook)))
    ws((SheetJS\nWorksheet))
  end
  csv(CSV\nstring)
  docs[[Documents\nArray]]
  file --> |readFile\n\n| wb
  wb --> |wb.Sheets\nselect sheet| ws
  ws --> |sheet_to_csv\n\n| csv
  csv --> |CSVLoader\n\n| docs
  linkStyle 0,1,2 color:blue,stroke:blue;
```

The SheetJS `readFile` method[^2] can read general workbooks. The method returns
a workbook object that conforms to the SheetJS data model[^3].

Workbook objects represent multi-sheet workbook files. They store individual
worksheet objects and other metadata.

Each worksheet in the workbook can be written to CSV text using the SheetJS
`sheet_to_csv`[^4] method.

For example, the following NodeJS script reads `pres.xlsx` and displays CSV rows
from the first worksheet:

```js title="Print CSV data from the first worksheet"
/* Load SheetJS Libraries */
import { readFile, set_fs, utils } from 'xlsx';

/* Load 'fs' for readFile support */
import * as fs from 'fs';
set_fs(fs);

/* Parse `pres.xlsx` */
const wb = readFile("pres.xlsx");

/* Print CSV rows from first worksheet */
const first_ws = wb.Sheets[wb.SheetNames[0]];
const csv = utils.sheet_to_csv(first_ws);
console.log(csv);
```

### Single Worksheet

For a single worksheet, a SheetJS pre-processing step can write the CSV rows to
file and the `CSVLoader` can load the newly written file.

<details open>
  <summary><b>Code example</b> (click to hide)</summary>

```js title="Pulling data from the first worksheet of a workbook"
import { CSVLoader } from "@langchain/community/document_loaders/fs/csv";
import { readFile, set_fs, utils } from 'xlsx';

/* Load 'fs' for readFile support */
import * as fs from 'fs';
set_fs(fs);

/* Parse `pres.xlsx`` */
const wb = readFile("pres.xlsx");

/* Generate CSV and write to `pres.xlsx.csv` */
const first_ws = wb.Sheets[wb.SheetNames[0]];
const csv = utils.sheet_to_csv(first_ws);
fs.writeFileSync("pres.xlsx.csv", csv);

/* Create documents with CSVLoader */
const loader = new CSVLoader("pres.xlsx.csv");
const docs = await loader.load();

console.log(docs);
// ...
```

</details>

### Workbook

A workbook is a collection of worksheets. Each worksheet can be exported to a
separate CSV. If the CSVs are written to a subfolder, a `DirectoryLoader`[^5]
can process the files in one step.

<details open>
  <summary><b>Code example</b> (click to hide)</summary>

In this example, the script creates a subfolder named `csv`. Each worksheet in
the workbook will be processed and the generated CSV will be stored to numbered
files. The first worksheet will be stored to `csv/0.csv`.

```js title="Pulling data from the each worksheet of a workbook"
import { CSVLoader } from "@langchain/community/document_loaders/fs/csv";
import { DirectoryLoader } from "langchain/document_loaders/fs/directory";
import { readFile, set_fs, utils } from 'xlsx';

/* Load 'fs' for readFile support */
import * as fs from 'fs';
set_fs(fs);

/* Parse `pres.xlsx`` */
const wb = readFile("pres.xlsx");

/* Create a folder `csv` */
try { fs.mkdirSync("csv"); } catch(e) {}

/* Generate CSV data for each worksheet */
wb.SheetNames.forEach((name, idx) => {
  const ws = wb.Sheets[name];
  const csv = utils.sheet_to_csv(ws);
  fs.writeFileSync(`csv/${idx}.csv`, csv);
});

/* Create documents with DirectoryLoader */
const loader = new DirectoryLoader("csv", {
  ".csv": (path) => new CSVLoader(path)
});
const docs = await loader.load();

console.log(docs);
// ...
```

</details>

## SheetJS Loader

The `CSVLoader` that ships with LangChain does not add any Document metadata and
does not generate any attributes. A custom loader can work around limitations in
the CSV tooling and potentially include metadata that has no CSV equivalent.

```mermaid
flowchart LR
  file[(Workbook\nXLSX/XLS)]
  subgraph SheetJS Structures
    wb(((SheetJS\nWorkbook)))
    ws((SheetJS\nWorksheet))
  end
  aoo[(Array of\nObjects)]
  docs[[Documents\nArray]]
  file --> |readFile\n\n| wb
  wb --> |wb.Sheets\nEach worksheet| ws
  ws --> |sheet_to_json\n\n| aoo
  aoo --> |new Document\nEach Row| docs
  linkStyle 0,1,2 color:blue,stroke:blue;
```

The demo [`LoadOfSheet` loader](pathname:///loadofsheet/loadofsheet.mjs) will
generate one Document per data row across all worksheets. It will also attempt
to build metadata and attributes for use in self-querying retrievers.

<details>
  <summary><b>Sample SheetJS Loader</b> (click to show)</summary>

This example loader pulls data from each worksheet. It assumes each worksheet
includes one header row and a number of data rows.

```js title="loadofsheet.mjs"
import { Document } from "@langchain/core/documents";
import { BufferLoader } from "langchain/document_loaders/fs/buffer";
import { read, utils } from "xlsx";

/**
 * Document loader that uses SheetJS to load documents.
 *
 * Each worksheet is parsed into an array of row objects using the SheetJS
 * `sheet_to_json` method and projected to a `Document`. Metadata includes
 * original sheet name, row data, and row index
 */
export default class LoadOfSheet extends BufferLoader {
  /** @type {import("langchain/chains/query_constructor").AttributeInfo[]}  */
  attributes = [];

  /**
   * Document loader that uses SheetJS to load documents.
   *
   * @param {string|Blob} filePathOrBlob Source Data
   */
  constructor(filePathOrBlob) {
    super(filePathOrBlob);
    this.attributes = [];
  }

  /**
   * Parse document
   *
   * NOTE: column labels in multiple sheets are not disambiguated!
   *
   * @param {Buffer} raw Raw data Buffer
   * @param {Document["metadata"]} metadata Document metadata
   * @returns {Promise<Document[]>} Array of Documents
   */
  async parse(raw, metadata) {
    /** @type {Document[]} */
    const result = [];

    this.attributes = [
      { name: "worksheet", description: "Sheet or Worksheet Name", type: "string" },
      { name: "rowNum", description: "Row index", type: "number" }
    ];

    const wb = read(raw, {type: "buffer", WTF:1});
    for(let name of wb.SheetNames) {
      const fields = {};
      const ws = wb.Sheets[name];
      if(!ws) return;

      const aoo = utils.sheet_to_json(ws);
      aoo.forEach((row, idx) => {
        result.push({
          pageContent: "Row " + (idx + 1) + " has the following content: \n" + Object.entries(row).map(kv => `- ${kv[0]}: ${kv[1]}`).join("\n") + "\n",
          metadata: {
            worksheet: name,
            rowNum: row["__rowNum__"],
            ...metadata,
            ...row
          }
        });
        Object.entries(row).forEach(([k,v]) => { if(v != null) (fields[k] || (fields[k] = {}))[v instanceof Date ? "date" : typeof v] = true } );
      });
      Object.entries(fields).forEach(([k,v]) => this.attributes.push({
        name: k, description: k, type: Object.keys(v).join(" or ")
      }));
    }

    return result;
  }
};
```

</details>

### From Text to Binary

Many libraries and platforms offer generic "text" loaders that process files
assuming the UTF8 encoding. This corrupts many spreadsheet formats including
XLSX, XLSB, XLSM and XLS.

:::note pass

This issue affects many JavaScript tools. Various demos cover workarounds:

- [ViteJS plugins](/docs/demos/static/vitejs#plugins) receive the relative path
to the workbook file and can read the file directly.

- [Webpack Plugins](/docs/demos/static/webpack#sheetjs-loader) have a special
option to instruct the library to pass raw binary data rather than text.

:::

The `CSVLoader` extends a special `TextLoader` that forces UTF8 text parsing.

There is a separate `BufferLoader` class, used by the PDF loader, that passes
the raw data using NodeJS `Buffer` objects.

<table>
  <thead><tr><th>Binary</th><th>Text</th></tr></thead>
  <tbody><tr><td>

```ts title="pdf.ts (structure)"
export class PDFLoader extends BufferLoader {
  // ...
  public async parse(
    raw: Buffer,
    metadata: Document["metadata"]
  ): Promise<Document[]> {
    // ...
  }
  // ...
}
```

  </td><td>

```ts title="csv.ts (structure)"
export class CSVLoader extends TextLoader {
  // ...
  protected async parse(
    raw: string

  ): Promise<string[]> {
    // ...
  }
  // ...
}
```

  </td></tr></tbody>
</table>

### NodeJS Buffers

The SheetJS `read` method supports NodeJS Buffer objects directly[^6]:

```js title="Parsing a workbook in a BufferLoader"
import { BufferLoader } from "langchain/document_loaders/fs/buffer";
import { read, utils } from "xlsx";

export default class LoadOfSheet extends BufferLoader {
  // ...
  async parse(raw, metadata) {
    // highlight-next-line
    const wb = read(raw, {type: "buffer"});
    // At this point, `wb` is a SheetJS workbook object
    // ...
  }
}
```

The `read` method returns a SheetJS workbook object[^7].

### Generating Content

The SheetJS `sheet_to_json` method[^8] returns an array of data objects whose
keys are drawn from the first row of the worksheet.

<table>
  <thead><tr><th>Spreadsheet</th><th>Array of Objects</th></tr></thead>
  <tbody><tr><td>

![`pres.xlsx` data](pathname:///pres.png)

</td><td>

```js
[
  { Name: "Bill Clinton", Index: 42 },
  { Name: "GeorgeW Bush", Index: 43 },
  { Name: "Barack Obama", Index: 44 },
  { Name: "Donald Trump", Index: 45 },
  { Name: "Joseph Biden", Index: 46 }
]
```

</td></tr></tbody></table>

The original `CSVLoader` wrote one row for each key-value pair. This text can be
generated by looping over the keys and values of the data row object. The
`Object.entries` helper function simplifies the conversion:

```js
function make_csvloader_doc_from_row_object(row) {
  return Object.entries(row).map(([k,v]) => `${k}: ${v}`).join("\n");
}
```

### Generating Documents

The loader must generate row objects for each worksheet in the workbook.

In the SheetJS data model, the workbook object has two relevant fields:

- `SheetNames` is an array of sheet names
- `Sheets` is an object whose keys are sheet names and values are sheet objects.

A `for..of` loop can iterate across the worksheets:

```js title="Looping over a workbook (skeleton)"
    const wb = read(raw, {type: "buffer", WTF:1});
    for(let name of wb.SheetNames) {
      const ws = wb.Sheets[name];
      const aoa = utils.sheet_to_json(ws);
      // at this point, `aoa` is an array of objects
    }
```

This simplified `parse` function uses the snippet from the previous section:

```js title="BufferLoader parse function (skeleton)"
  async parse(raw, metadata) {
    /* array to hold generated documents */
    const result = [];

    /* read workbook */
    const wb = read(raw, {type: "buffer", WTF:1});

    /* loop over worksheets */
    for(let name of wb.SheetNames) {
      const ws = wb.Sheets[name];
      const aoa = utils.sheet_to_json(ws);

      /* loop over data rows */
      aoa.forEach((row, idx) => {
        /* generate a new document and add to the result array */
        result.push({
          pageContent: Object.entries(row).map(([k,v]) => `${k}: ${v}`).join("\n")
        });
      });
    }

    return result;
  }
```

### Metadata and Attributes

It is strongly recommended to generate additional metadata and attributes for
self-query retrieval applications.

<details>
  <summary><b>Implementation Details</b> (click to show)</summary>

**Metadata**

Metadata is attached to each document object. The following example appends the
raw row data to the document metadata:

```js title="Document with metadata (snippet)"
        /* generate a new document and add to the result array */
        result.push({
          pageContent: Object.entries(row).map(([k,v]) => `${k}: ${v}`).join("\n"),
          metadata: {
            worksheet: name, // name of the worksheet
            rowNum: idx, // data row index
            ...row // raw row data
          }
        });
```

**Attributes**

Each attribute object specifies three properties:

- `name` corresponds to the field in the document metadata
- `description` is a description of the field
- `type` is a description of the data type.

While looping through data rows, a simple type check can keep track of the data
type for each column:

```js title="Tracking column types (sketch)"
    for(let name of wb.SheetNames) {
      /* track column types */
      const fields = {};
      // ...

      aoo.forEach((row, idx) => {
        result.push({/* ... */});
        /* Check each property */
        Object.entries(row).forEach(([k,v]) => {
          /* Update fields entry to reflect the new data point */
          if(v != null) (fields[k] || (fields[k] = {}))[v instanceof Date ? "date" : typeof v] = true
        });
      });
      // ...
    }
```

Attributes can be generated after writing the worksheet data. Storing attributes
in a loader property will make it accessible to scripts that use the loader.

```js title="Adding Attributes to a Loader (sketch)"
export default class LoadOfSheet extends BufferLoader {
  // highlight-next-line
  attributes = [];
  // ...

  async parse(raw, metadata) {
    // Add the worksheet name and row index attributes
    // highlight-start
    this.attributes = [
      { name: "worksheet", description: "Sheet or Worksheet Name", type: "string" },
      { name: "rowNum", description: "Row index", type: "number" }
    ];
    // highlight-end
    const wb = read(raw, {type: "buffer", WTF:1});
    for(let name of wb.SheetNames) {
      // highlight-next-line
      const fields = {};
      // ...
      const aoo = utils.sheet_to_json(ws);
      aoo.forEach((row, idx) => {
        result.push({/* ... */});
        /* Check each property */
        Object.entries(row).forEach(([k,v]) => {
          /* Update fields entry to reflect the new data point */
          if(v != null) (fields[k] || (fields[k] = {}))[v instanceof Date ? "date" : typeof v] = true
        });
      });
      /* Add one attribute per metadata field */
      // highlight-start
      Object.entries(fields).forEach(([k,v]) => this.attributes.push({
        name: k, description: k,
        /* { number: true, string: true } -> "number or string" */
        type: Object.keys(v).join(" or ")
      }));
      // highlight-end
    }
    // ...
  }
```

</details>

## SheetJS Loader Demo

The demo performs the query "Which rows have over 40 miles per gallon?" against
a [sample cars dataset](pathname:///cd.xls) and displays the results.

:::caution pass

This demo was tested using the ChatQA-1.5 model[^9] in Ollama[^10].

The tested model requires 9.2GB VRAM. It is strongly recommended to run the demo
on a newer Apple Silicon Mac or a PC with an Nvidia GPU with at least 12GB VRAM.

:::

0) Create a new project:

```bash
mkdir sheetjs-loader
cd sheetjs-loader
npm init -y
```

1) Download the demo scripts:

- [`loadofsheet.mjs`](pathname:///loadofsheet/loadofsheet.mjs)
- [`query.mjs`](pathname:///loadofsheet/query.mjs)

```bash
curl -LO https://docs.sheetjs.com/loadofsheet/query.mjs
curl -LO https://docs.sheetjs.com/loadofsheet/loadofsheet.mjs
```

2) Install the SheetJS NodeJS module:

<CodeBlock language="bash">{`\
npm i --save https://cdn.sheetjs.com/xlsx-${current}/xlsx-${current}.tgz`}
</CodeBlock>

3) Install LangChain and HNSWLib dependencies:

```bash
npm i --save @langchain/community@0.2.0 @langchain/core@0.2.6 langchain@0.2.5 hnswlib-node@3.0.0 peggy@3.0.2
```

4) Download the [cars dataset](pathname:///cd.xls):

```bash
curl -LO https://docs.sheetjs.com/cd.xls
```

5) Install the `llama3-chatqa:8b-v1.5-q8_0` model using Ollama:

```bash
ollama pull llama3-chatqa:8b-v1.5-q8_0
```

:::note pass

If the command cannot be found, install Ollama[^10] and run the command in a new
terminal window.

:::

6) Run the demo script

```bash
node query.mjs
```

The demo performs the query "Which rows have over 40 miles per gallon?". It will
print the following nine results:

```js title="Expected output"
{ Name: 'volkswagen rabbit custom diesel', MPG: 43.1 }
{ Name: 'vw rabbit c (diesel)', MPG: 44.3 }
{ Name: 'renault lecar deluxe', MPG: 40.9 }
{ Name: 'honda civic 1500 gl', MPG: 44.6 }
{ Name: 'datsun 210', MPG: 40.8 }
{ Name: 'vw pickup', MPG: 44 }
{ Name: 'mazda glc', MPG: 46.6 }
{ Name: 'vw dasher (diesel)', MPG: 43.4 }
{ Name: 'vw rabbit', MPG: 41.5 }
```

To find the expected results:

- Open the `cd.xls` spreadsheet in Excel
- Select Home > Sort &amp; Filter > Filter in the Ribbon
- Select the filter option for column B (`Miles_per_Gallon`)
- In the popup, select "Greater Than" in the Filter dropdown and type 40

The filtered results should match the following screenshot:

![Expected Results](pathname:///loadofsheet/expected.png)

[^1]: See ["How to load CSV data"](https://js.langchain.com/v0.2/docs/how_to/document_loader_csv) in the LangChain documentation
[^2]: See [`readFile` in "Reading Files"](/docs/api/parse-options)
[^3]: See ["SheetJS Data Model"](/docs/csf/)
[^4]: See [`sheet_to_csv` in "CSV and Text"](/docs/api/utilities/csv#delimiter-separated-output)
[^5]: See ["Folders with multiple files"](https://js.langchain.com/v0.2/docs/integrations/document_loaders/file_loaders/directory/) in the LangChain documentation
[^6]: See ["Supported Output Formats" type in "Writing Files"](/docs/api/write-options#supported-output-formats)
[^7]: See ["Workbook Object"](/docs/csf/book)
[^8]: See [`sheet_to_json` in "Utilities"](/docs/api/utilities/array#array-output)
[^9]: See [the official ChatQA website](https://chatqa-project.github.io/) for the ChatQA paper and other model details.
[^10]: See [the official Ollama website](https://ollama.com/download) for installation instructions.