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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchRead the JSON in your test process, create the browser context, and register context.addInitScript() before opening or navigating the page. The init script copies each JSON entry into window.sessionStorage before the application’s scripts run. Playwright’s normal storageState mechanism does not persist session storage; this is the documented workaround in the authentication guide.
The working pattern
Assume playwright/.auth/session.json contains an object such as:
{
"onboardingComplete": "true",
"draftId": "abc-123",
"preferences": "{"theme":"dark"}"
}Load and parse that file, then pass the object to an init script. This complete TypeScript example limits writes to the intended hostname:
import fs from 'node:fs';
import { chromium } from 'playwright';
const sessionStorage = JSON.parse(
fs.readFileSync('playwright/.auth/session.json', 'utf-8'),
);
const browser = await chromium.launch();
const context = await browser.newContext();
await context.addInitScript((storage) => {
if (window.location.hostname !== 'example.com') return;
for (const [key, value] of Object.entries(storage)) {
window.sessionStorage.setItem(key, String(value));
}
}, sessionStorage);
const page = await context.newPage();
await page.goto('https://example.com/');
await browser.close();The same sequence works in JavaScript after removing the TypeScript-only annotations. Register the script before newPage() and before any navigation that needs the values. addInitScript runs after a document is created but before that document’s own scripts execute, and it applies to pages created in the context and to subsequent navigations. The BrowserContext API reference also documents that init scripts can run in child frames.
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Prepare the JSON correctly
Use an object of key/value pairs
The code expects the top-level JSON value to be an object. An array, a nested export such as {"sessionStorage": {...}}, or malformed JSON will not produce the entries the loop expects. If your export is nested, select the correct property before calling addInitScript:
const file = JSON.parse(fs.readFileSync(fileName, 'utf8'));
const sessionStorage = file.sessionStorage ?? file;Remember that Web Storage stores strings
sessionStorage.setItem() stores strings. Calling String(value) turns a number into text and turns a boolean into "true" or "false". For objects and arrays, that conversion becomes "[object Object]", which is usually not what an application expects. Serialize structured values deliberately:
await context.addInitScript((storage) => {
if (window.location.hostname !== 'example.com') return;
for (const [key, value] of Object.entries(storage)) {
const text = value !== null && typeof value === 'object'
? JSON.stringify(value)
: String(value);
window.sessionStorage.setItem(key, text);
}
}, sessionStorage);If the application expects JSON, it will normally call JSON.parse(sessionStorage.getItem('preferences')); store the corresponding JSON string, not a JavaScript object.
Scope the script to the right origin
Session storage is isolated by origin (scheme, host, and port) and by the page’s storage context. A value seeded for https://example.com is not automatically available at another scheme, port, or unrelated host. The official example uses a hostname check:
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const allowedOrigins = new Set([
'https://staging.example.com',
'https://app.example.com',
]);
await context.addInitScript((storage, origins) => {
if (!origins.includes(window.location.origin)) return;
for (const [key, value] of Object.entries(storage)) {
window.sessionStorage.setItem(key, String(value));
}
}, sessionStorage, [...allowedOrigins]);Do not seed every page in a context indiscriminately. A broad script can leak test data into a third-party frame or a different application visited later. If a site uses several subdomains, decide explicitly whether each exact origin should receive the values. Child frames are also candidates for init-script execution, so the guard matters there as well.
Use it with Playwright Test
In the test runner, install the script on the supplied context fixture before navigating:
import { test as base, expect } from '@playwright/test';
import fs from 'node:fs';
type SessionStorage = Record<string, unknown>;
const sessionStorage: SessionStorage = JSON.parse(
fs.readFileSync('playwright/.auth/session.json', 'utf8'),
);
export const test = base.extend({
page: async ({ context, page }, use) => {
await context.addInitScript((storage) => {
if (window.location.hostname !== 'example.com') return;
for (const [key, value] of Object.entries(storage)) {
window.sessionStorage.setItem(key, String(value));
}
}, sessionStorage);
await use(page);
},
});
export { expect };
// example.spec.ts
import { test, expect } from './fixtures';
test('opens with the saved session data', async ({ page }) => {
await page.goto('https://example.com/dashboard');
await expect(page).toHaveURL(/dashboard/);
});Alternatively, call context.addInitScript in a beforeEach hook before page.goto. The runner’s storageState option remains useful for cookies and supported origin storage, but it is not a session-storage snapshot. See the TestOptions API for the runner option.
Capture a session-storage file for later reuse
The official authentication guide demonstrates the reverse operation: evaluate sessionStorage in the page, serialize it, and save the result. A minimal capture looks like this:
import fs from 'node:fs';
import { chromium } from 'playwright';
const browser = await chromium.launch();
const context = await browser.newContext();
const page = await context.newPage();
await page.goto('https://example.com/');
const json = await page.evaluate(() => JSON.stringify(sessionStorage));
fs.mkdirSync('playwright/.auth', { recursive: true });
fs.writeFileSync('playwright/.auth/session.json', json);
await browser.close();This is an advanced workflow. The documentation notes that session storage is rarely used for signed-in state; authentication is often represented by cookies, local storage, or another mechanism. Inspect the application before deciding that this file is the state you need.
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How this differs from storageState
| State or API | What it is suitable for | Session-storage JSON loaded automatically? |
|---|---|---|
storageState |
Reusable cookies and the storage types documented by Playwright’s authentication flow | No |
context.addInitScript |
Writing selected values into window.sessionStorage before application code |
Yes, when you read and pass the JSON yourself |
page.evaluate |
Reading or changing storage after a document exists | It is too late for code that already read storage during startup |
Playwright’s authentication documentation explicitly says it has no API to persist session storage and supplies the init-script approach. Newer adjacent APIs do not remove that limitation: the current references list BrowserContext.setStorageState in v1.59, IndexedDB capture in v1.51, WebStorage APIs in v1.61, WebAuthn credential capture in v1.61, and OPFS capture in v1.63. Those version numbers describe the referenced APIs; they do not turn storageState into a session-storage importer.
Security and maintenance precautions
- Treat the JSON as a credential. Tokens, identifiers, or setup state can impersonate a user. Playwright warns against committing saved authentication state to public or private repositories; apply the same rule to session-storage files.
- Put the file in an ignored directory such as
playwright/.auth, restrict its permissions in CI, and provide it through a secret or setup step. - Use a separate test account and short-lived data where possible. Delete or rotate exports when a run is complete.
- Validate the shape before injecting it. Reject unexpected keys rather than silently loading an entire untrusted JSON document.
- Keep one context per independent state. Session storage is context/page state, so reusing a context between unrelated tests can create order-dependent failures.
Troubleshooting
The application still behaves as logged out
Check whether login is actually held in cookies, local storage, IndexedDB, or a server-side session. Session storage alone may not be sufficient. If the app redirects before your expected origin, the guard may never match; log page.url() and inspect the final origin.
The values appear only after a reload
The script was probably registered after the first navigation, or the values were written with page.evaluate after startup. Register addInitScript before newPage() or before the navigation, then create a fresh page or reload.
Nothing is written
Print the parsed keys in the Node process and verify the file path and encoding. In the browser, check window.location.origin and compare it with your guard. A hostname-only guard will not distinguish ports; use an exact origin set when necessary.
Objects come back as [object Object]
That is JavaScript’s ordinary string conversion. Store nested values with JSON.stringify and parse them in the application, or export the exact string representation the application expects.
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A frame receives the wrong values
Init scripts can run in child frames. Narrow the origin check and, if necessary, avoid navigating third-party frames in the same context. Do not assume a top-level-page check protects every frame.
Parallel tests interfere
Do not mutate shared JSON or share a context between tests that require different session data. Create isolated contexts, and make each fixture deterministic. A cache of the parsed file in the Node process is safe; a shared browser context is often not.
The JSON file is rejected or empty
Run JSON.parse on the file in the test process and report the parse error before launching the browser. Confirm that the capture step completed after the page had initialized its storage and that the file was not truncated by a failed setup job.
Performance and reliability notes
The init script performs one synchronous-looking loop in the page for each stored key. Keep the export limited to values the test actually needs; large blobs increase document-start work and make failures harder to diagnose. Registering one script per context is preferable to repeatedly adding scripts for every navigation. If a test needs different state, create another context or use a separate fixture rather than conditionally overwriting keys after the app has started.
For reproducibility, record the target origin, the export timestamp, and the application version alongside the secret file outside version control. Expired tokens and server-side sessions can make a technically correct injection fail, so refresh the state through the application’s supported login/setup flow when it expires.
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One GET request is enough:
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Practical checklist
- Parse a top-level JSON object in the Node test process.
- Register
context.addInitScriptbefore creating or navigating the page. - Guard the exact origin(s) that should receive the data.
- Convert values to the strings the application expects.
- Use
storageStateseparately for cookies and other supported state. - Keep session files secret and isolate browser contexts between independent tests.
Frequently Asked Questions
Can I pass the JSON filename directly to Playwright?
No. Read and parse the file in your test process, then pass the resulting object to an init script.
Will this work for every tab in a context?
The script is installed for pages created in that context and for their navigations, subject to your origin guard.
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Only if the application actually stores the required state there. Playwright’s guide notes that signed-in state is often represented by cookies or other storage instead.
Can I use a hostname guard for multiple ports?
Use an exact window.location.origin allowlist when ports, schemes, or subdomains differ.
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