javascript - How to save and restore a File object in local storage

Javascript - How to save and restore a File object in local storage

In JavaScript, you cannot directly save a File object (representing a file from the user's system) to local storage due to security restrictions and the fact that File objects are not serializable. However, you can achieve a similar result by following these steps:

Saving a File Object to Local Storage

  1. Convert the File Object to a Blob: Use a FileReader to read the File object as a Blob.

  2. Convert Blob to Base64: Convert the Blob to a Base64 string using FileReader.

  3. Store Base64 String in Local Storage: Save the Base64 string representation of the file in local storage.

Example: Saving a File Object

function saveFileToLocalStorage(file) { if (!file) return; // Step 1: Read the file as a Blob let reader = new FileReader(); reader.readAsDataURL(file); // Step 2: Convert Blob to Base64 reader.onload = function () { let base64String = reader.result.split(',')[1]; // Step 3: Store Base64 string in local storage localStorage.setItem('savedFile', base64String); console.log('File saved to local storage.'); }; }

Restoring a File Object from Local Storage

To restore the file from local storage, you need to reverse the process:

  1. Retrieve Base64 String from Local Storage: Retrieve the Base64 string from local storage.

  2. Convert Base64 String to Blob: Convert the Base64 string back to a Blob.

  3. Create a File Object: Create a File object from the Blob.

Example: Restoring a File Object

function restoreFileFromLocalStorage() { // Step 1: Retrieve Base64 string from local storage let base64String = localStorage.getItem('savedFile'); if (!base64String) { console.log('No file found in local storage.'); return; } // Step 2: Convert Base64 string to Blob let blob = b64toBlob(base64String, 'application/octet-stream'); // Step 3: Create a File object (you may need to set the filename) let file = new File([blob], 'restoredFile.txt', { type: 'text/plain' }); console.log('File restored from local storage:', file); } // Function to convert Base64 string to Blob function b64toBlob(b64Data, contentType = '', sliceSize = 512) { const byteCharacters = atob(b64Data); const byteArrays = []; for (let offset = 0; offset < byteCharacters.length; offset += sliceSize) { const slice = byteCharacters.slice(offset, offset + sliceSize); const byteNumbers = new Array(slice.length); for (let i = 0; i < slice.length; i++) { byteNumbers[i] = slice.charCodeAt(i); } const byteArray = new Uint8Array(byteNumbers); byteArrays.push(byteArray); } const blob = new Blob(byteArrays, { type: contentType }); return blob; }

Explanation:

  • saveFileToLocalStorage(file): Converts the File object to a Base64 string and stores it in local storage under the key 'savedFile'.

  • restoreFileFromLocalStorage(): Retrieves the Base64 string from local storage, converts it back to a Blob, and then creates a File object.

  • b64toBlob(b64Data, contentType = '', sliceSize = 512): Helper function to convert a Base64 string to a Blob. It slices the Base64 data into smaller chunks and creates a Uint8Array to form the Blob.

Important Considerations:

  • Security and Size Limits: Local storage has size limits (typically 5-10MB per domain) and should not be used for large files due to performance and storage constraints.

  • Base64 Overhead: Base64 encoding increases the size of the file by approximately 33% due to encoding overhead.

  • Cross-Origin Restrictions: Local storage is domain-specific. You cannot access local storage items from a different domain or protocol (e.g., http vs https).

By following these steps, you can effectively save and restore a File object using local storage in JavaScript, accommodating the limitations and considerations mentioned above.

Examples

  1. Save File object to local storage

    • Description: Save a File object to local storage using JSON serialization.
    • Code:
      const fileInput = document.getElementById('fileInput'); fileInput.addEventListener('change', function(event) { const file = event.target.files[0]; const reader = new FileReader(); reader.onload = function(event) { const fileData = event.target.result; localStorage.setItem('savedFile', JSON.stringify({ name: file.name, type: file.type, data: fileData })); }; reader.readAsDataURL(file); });

  2. Restore File object from local storage

    • Description: Retrieve and restore a File object from local storage.
    • Code:
      const savedFile = localStorage.getItem('savedFile'); if (savedFile) { const fileData = JSON.parse(savedFile); const blob = new Blob([fileData.data], { type: fileData.type }); const restoredFile = new File([blob], fileData.name, { type: fileData.type }); console.log('Restored File:', restoredFile); } else { console.log('No file data found in local storage.'); }

  3. Convert File to Base64 and save in local storage

    • Description: Convert a File object to Base64 and store it in local storage for later retrieval.
    • Code:
      const fileInput = document.getElementById('fileInput'); fileInput.addEventListener('change', function(event) { const file = event.target.files[0]; const reader = new FileReader(); reader.onload = function(event) { const base64Data = event.target.result.split(',')[1]; localStorage.setItem('savedBase64File', base64Data); }; reader.readAsDataURL(file); });

  4. Restore File from Base64 stored in local storage

    • Description: Retrieve and restore a File object from Base64 data stored in local storage.
    • Code:
      const savedBase64File = localStorage.getItem('savedBase64File'); if (savedBase64File) { const byteCharacters = atob(savedBase64File); const byteNumbers = new Array(byteCharacters.length); for (let i = 0; i < byteCharacters.length; i++) { byteNumbers[i] = byteCharacters.charCodeAt(i); } const byteArray = new Uint8Array(byteNumbers); const blob = new Blob([byteArray], { type: 'application/octet-stream' }); const restoredFile = new File([blob], 'restoredFile'); console.log('Restored File:', restoredFile); } else { console.log('No Base64 file data found in local storage.'); }

  5. Save File metadata to local storage

    • Description: Store metadata of a File object (name, type) in local storage.
    • Code:
      const fileInput = document.getElementById('fileInput'); fileInput.addEventListener('change', function(event) { const file = event.target.files[0]; localStorage.setItem('savedFileName', file.name); localStorage.setItem('savedFileType', file.type); });

  6. Restore File metadata from local storage

    • Description: Retrieve and restore metadata of a File object from local storage.
    • Code:
      const savedFileName = localStorage.getItem('savedFileName'); const savedFileType = localStorage.getItem('savedFileType'); if (savedFileName && savedFileType) { console.log('Restored File Name:', savedFileName); console.log('Restored File Type:', savedFileType); } else { console.log('No file metadata found in local storage.'); }

  7. Save File object using sessionStorage

    • Description: Use sessionStorage to save a File object temporarily.
    • Code:
      const fileInput = document.getElementById('fileInput'); fileInput.addEventListener('change', function(event) { const file = event.target.files[0]; sessionStorage.setItem('sessionFile', JSON.stringify({ name: file.name, type: file.type, data: file })); });

  8. Restore File object from sessionStorage

    • Description: Retrieve and restore a File object from sessionStorage.
    • Code:
      const sessionFile = sessionStorage.getItem('sessionFile'); if (sessionFile) { const fileData = JSON.parse(sessionFile); const restoredFile = new File([fileData.data], fileData.name, { type: fileData.type }); console.log('Restored File:', restoredFile); } else { console.log('No session file data found.'); }

  9. Save File object using IndexedDB

    • Description: Store a File object in IndexedDB for more robust storage.
    • Code:
      const fileInput = document.getElementById('fileInput'); fileInput.addEventListener('change', function(event) { const file = event.target.files[0]; const dbPromise = window.indexedDB.open('fileStorage', 1); dbPromise.onsuccess = function(event) { const db = event.target.result; const transaction = db.transaction(['files'], 'readwrite'); const store = transaction.objectStore('files'); store.put(file, 'file'); }; });

  10. Restore File object from IndexedDB

    • Description: Retrieve and restore a File object from IndexedDB.
    • Code:
      const dbPromise = window.indexedDB.open('fileStorage', 1); dbPromise.onsuccess = function(event) { const db = event.target.result; const transaction = db.transaction(['files'], 'readonly'); const store = transaction.objectStore('files'); const request = store.get('file'); request.onsuccess = function(event) { const file = event.target.result; console.log('Restored File:', file); }; };

More Tags

webdriver android-json apache-kafka-security file-get-contents struts2 azure-pipelines-build-task feature-detection drawing entity-framework-core-2.1 dropshadow

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