[Feature] improve CsvLoader & clean code (#3830)

* Improve CSV Loader

* Improve S3 Loaders

---------

Co-authored-by: Henry <hzj94@hotmail.com>

packages/components/nodes/documentloaders/Csv/CsvLoader.ts

@@ -0,0 +1,74 @@
+import { TextLoader } from 'langchain/document_loaders/fs/text'
+import Papa from 'papaparse'
+
+type CSVLoaderOptions = {
+    // Return specifific column from key (string) or index (integer)
+    column?: string | number
+    // Force separator (default: auto detect)
+    separator?: string
+}
+
+/**
+ * A class that extends the TextLoader class. It represents a document
+ * loader that loads documents from a CSV file. It has a constructor that
+ * takes a `filePathOrBlob` parameter representing the path to the CSV
+ * file or a Blob object, and an optional `options` parameter of type
+ * `CSVLoaderOptions` or a string representing the column to use as the
+ * document's pageContent.
+ */
+export class CSVLoader extends TextLoader {
+    protected options: CSVLoaderOptions = {}
+
+    constructor(filePathOrBlob: ConstructorParameters<typeof TextLoader>[0], options?: CSVLoaderOptions | string) {
+        super(filePathOrBlob)
+
+        if (typeof options === 'string') {
+            this.options = { column: options }
+        } else {
+            this.options = options ?? this.options
+        }
+    }
+    /**
+     * A protected method that parses the raw CSV data and returns an array of
+     * strings representing the pageContent of each document. It uses the
+     * `papaparse` to parse the CSV data. If
+     * the `column` option is specified, it checks if the column exists in the
+     * CSV file and returns the values of that column as the pageContent. If
+     * the `column` option is not specified, it converts each row of the CSV
+     * data into key/value pairs and joins them with newline characters.
+     * @param raw The raw CSV data to be parsed.
+     * @returns An array of strings representing the pageContent of each document.
+     */
+    async parse(raw: string): Promise<string[]> {
+        const { column, separator } = this.options
+
+        const {
+            data: parsed,
+            meta: { fields = [] }
+        } = Papa.parse<{ [K: string]: string }>(raw.trim(), {
+            delimiter: separator,
+            header: true
+        })
+
+        if (column !== undefined) {
+            if (!fields.length) {
+                throw new Error(`Unable to resolve fields from header.`)
+            }
+
+            let searchIdx = column
+
+            if (typeof column == 'number') {
+                searchIdx = fields[column]
+            }
+
+            if (!fields.includes(searchIdx as string)) {
+                throw new Error(`Column ${column} not found in CSV file.`)
+            }
+
+            // Note TextLoader will raise an exception if the value is null.
+            return parsed.map((row) => row[searchIdx])
+        }
+
+        return parsed.map((row) => fields.map((key) => `${key.trim() || '_0'}: ${row[key]?.trim()}`).join('\n'))
+    }
+}