Creating a text file using javascript

Per the documentation on the Document object, you need to base64 encode the data stored in the body field.

Body - Type: base64, Description: Encoded file data.

Encoded Data

The API sends and receives the binary file data encoded as a base64 data type. Prior to creating a record, clients must encode the binary file data as base64. Upon receiving an API response, clients must decode the base64 data to binary (this conversion is usually handled for you by the SOAP client).

There are quite a few ways to do this. If this code is executing in a newer browser you can use the btoa() method available on the window object. If your browsers do not support the btoa() method , you'll probably need to find a library which you like that can handle this encoding instead.

function createDocument(body) {
    // console.log("body: " + body);
    var document = new sforce.SObject('Document');
    var now = new Date();
    document.Name = now + '_DataStore.json';

    // base64 encode the body string
    document.Body = window.btoa(body);

    document.FolderId = '005d0000000fLdI';
    document.ContentType = 'application/json';
    result = sforce.connection.create([document]);
    if (!result[0].getBoolean("success")) {
        alert("Failed to create document");
    } else {
        window.location.href = '/' + result[0].id;
    }
}