--- title: Local Storage API pagination_prev: demos/data/index pagination_next: demos/cloud/index sidebar_custom_props: summary: Reading and writing data in an in-browser Key-Value store --- 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 :::note Tested Deployments Each browser demo was tested in the following environments: | Browser | Date | |:------------|:-----------| | Chrome 119 | 2023-11-30 | | Safari 16.6 | 2023-09-17 | ::: ## Row Objects Consider the following array of objects of data: ```js [ { 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: ```js 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: ```js 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 example will fetch <https://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: ![Local Storage view in Developer Tools](pathname:///storageapi/lstorage.png) :::caution pass This example is for illustration purposes. If array of objects is available, it is strongly recommended to convert that array to a worksheet directly. ::: <details><summary><b>Live Demo</b> (click to show)</summary> ```jsx live function SheetJStorage() { const [url, setUrl] = React.useState("https://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> </> ); } ``` </details> ## Simple Strings The ["Row Objects" approach](#row-objects) 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: ```js [ [ "b", "Logical" ], [ "n", "Numeric" ], [ "s", "Textual" ] ] ``` #### Exporting Storage :::note pass 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: ```js 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: ```js 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. <details><summary><b>Live Demo</b> (click to show)</summary> ```jsx live 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> </> ); } ``` </details>