Local Storage API

The Storage API, encompassing localStorage and sessionStorage, describes simple key-value stores that only support string values and keys.

This demo covers two common use patterns:

• "Row Objects" shows a simple convention for loading and storing row objects
• "Simple Strings" discusses how to persist and recover a raw Storage

Tested Deployments

Each browser demo was tested in the following environments:

Browser	Date
Chrome 122	2024-03-25
Safari 17.3	2024-03-12

Row Objects

Consider the following array of objects of data:

[
  { Name: "Barack Obama", Index: 44 },
  { Name: "Donald Trump", Index: 45 },
  { Name: "Joseph Biden", Index: 46 }
]

Storage API expects values to be strings. The simplest approach is to stringify row objects using JSON.stringify and store using the row index as a key:

Key	Value
0	{"Name":"Barack Obama","Index":44}
1	{"Name":"Donald Trump","Index":45}
2	{"Name":"Joseph Biden","Index":46}

Importing Data

Starting from a worksheet, XLSX.utils.sheet_to_json generates an array of row objects. localStorage.setItem will store data in Local Storage:

function sheet_to_localStorage(worksheet) {
  const aoo = XLSX.utils.sheet_to_json(worksheet);
  for(let i = 0; i < aoo.length; ++i) {
    localStorage.setItem(i, JSON.stringify(aoo[i]));
  }
}

Exporting Data

localStorage.length returns the total number of entries. A simple for loop can cover the keys (integers from 0 to localStorage.length - 1 inclusive)

localStorage.getItem will load the stringified data from the Local Storage. A new array of objects can be constructed by using JSON.parse and pushing to an array. XLSX.utils.json_to_sheet can create a new worksheet from that array:

function localStorage_to_sheet() {
  const aoo = [];
  for(let i = 0; i < localStorage.length; ++i) {
    aoo.push(JSON.parse(localStorage.getItem(i)));
  }
  return XLSX.utils.json_to_sheet(aoo);
}

Live Demo

This demo will fetch https://docs.sheetjs.com/pres.numbers, fill localStorage with rows, then generate a worksheet from the rows and write to a new file.

After saving the exported file, the Local Storage can be inspected in the "Local Storage" section of the "Application" Tab of Developer Tools:

This example is for illustration purposes. If array of objects is available, it is strongly recommended to convert that array to a worksheet directly.

Live Demo (click to show)

Result

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Live Editor

function SheetJStorage() {
  const [url, setUrl] = React.useState("https://docs.sheetjs.com/pres.numbers");
  const set_url = (evt) => setUrl(evt.target.value);
  const [out, setOut] = React.useState("");
  const xport = React.useCallback(async() => {
    // get first worksheet data as array of objects
    const wb = XLSX.read(await (await fetch(url)).arrayBuffer());
    const aoo = XLSX.utils.sheet_to_json(wb.Sheets[wb.SheetNames[0]]);

    // reset and populate localStorage
    localStorage.clear();
    for(var i = 0; i < aoo.length; ++i) localStorage.setItem(i, JSON.stringify(aoo[i]));

    // create new array of objects from localStorage
    const new_aoo = [];
    for(var i = 0; i < localStorage.length; ++i) {
      const row = JSON.parse(localStorage.getItem(i));
      new_aoo.push(row);
    }

    setOut(`Number of rows in LocalStorage: ${localStorage.length}`);

    // create and export workbook
    const new_ws = XLSX.utils.json_to_sheet(new_aoo);
    const new_wb = XLSX.utils.book_new();
    XLSX.utils.book_append_sheet(new_wb, new_ws, "Sheet1");
    XLSX.writeFile(new_wb, "SheetJStorage.xlsx");
  });

  return ( <>
    {out && ( <><a href={url}>{url}</a><pre>{out}</pre></> )}
    <b>URL: </b><input type="text" value={url} onChange={set_url} size="50"/>
    <br/><button onClick={xport}><b>Fetch!</b></button>
  </> );
}

Simple Strings

The "Row Objects" approach is strongly recommended when trying to store or recover arrays of row objects.

When the goal is to save an existing Storage, the general representation is an array of pairs. Consider the following data in Local Storage:

Key	Value
"b"	"Logical"
"n"	"Numeric"
"s"	"Textual"

The natural representation is an array of arrays:

[
  [ "b", "Logical" ],
  [ "n", "Numeric" ],
  [ "s", "Textual" ]
]

Exporting Storage

Web Storage iteration order is not defined. By using indices as keys, the row objects approach has an ordering. That does not apply to the general case.

In modern browsers, Object.entries will generate an array of key/value pairs. XLSX.utils.aoa_to_sheet will interpret that array as a worksheet with 2 cols:

function localStorage_to_ws() {
  const aoa = Object.entries(localStorage);
  return XLSX.utils.aoa_to_sheet(aoa);
}

Importing Storage

In the other direction, the worksheet is assumed to store keys in column A and values in column B. XLSX.utils.sheet_to_json with the header: 1 option will generate key/value pairs that can be assigned to a storage:

function ws_to_localStorage(ws) {
  const aoa = XLSX.utils.sheet_to_json(ws, { header: 1 });
  aoa.forEach(([key, val]) => localStorage.setItem(key, val));
}

Live Demo

This example fills localStorage with 10 random keys and 10 random values, generates a worksheet from the data and writes to a new file.

Live Demo (click to show)

Result

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Live Editor

function SheetJSRandomStorage() {
  const [out, setOut] = React.useState("");
  const [rows, setRows] = React.useState([]);
  const xport = React.useCallback(async() => {
    // reset and populate localStorage
    localStorage.clear();
    var data = [];
    for(let i = 0, last = 0; i < 10; ++i) {
      var k = ((Math.random() * 20)|0) + last;
      var v = (Math.random() * 16777216).toString(36);
      localStorage.setItem(k, v);
      data.push([k,v]);
      last = k;
    }
    setRows(Object.entries(localStorage));

    // create new worksheet from localStorage
    const aoa = Object.entries(localStorage);
    const new_ws = XLSX.utils.aoa_to_sheet(aoa);

    // create and export workbook
    const new_wb = XLSX.utils.book_new();
    XLSX.utils.book_append_sheet(new_wb, new_ws, "Sheet1");
    XLSX.writeFile(new_wb, "SheetJSRandomStorage.xlsx");
  });
  return ( <>
    {out && ( <><a href={url}>{url}</a><pre>{out}</pre></> )}
    {rows.length && (<table><tr><th>Key</th><th>Value</th></tr>
      {rows.map(([k,v]) => (<tr><td>{k}</td><td>{v}</td></tr>))}
    </table>) || null}
    <br/><button onClick={xport}><b>Export!</b></button>
  </> );
}