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docs(core-concepts): some nits (#1827)

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docs/core-concepts.md
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@ -24,16 +24,12 @@ const browser = await chromium.launch({ headless: false });
await browser.close(); await browser.close();
``` ```
After launching a browser, Playwright establishes a **bi-directional RPC channel**
between the Playwright script executing in Node.js and the browser instance.
Read more about [execution contexts](#execution-contexts).
Launching a browser instance can be expensive, and Playwright is designed to Launching a browser instance can be expensive, and Playwright is designed to
maximize what a single instance can do through multiple browser contexts. maximize what a single instance can do through multiple browser contexts.
### Browser Context ### Browser Context
A [`BrowserContext`](../api.md#class-browsercontext) is an isolated incognito A [`BrowserContext`](../api.md#class-browsercontext) is an isolated incognito-alike
session within a browser instance. Browser contexts are fast and cheap to create. session within a browser instance. Browser contexts are fast and cheap to create.
Browser contexts can be used to parallelize isolated test executions. Browser contexts can be used to parallelize isolated test executions.
@ -61,7 +57,7 @@ const context = await browser.newContext({
### Pages and Frames ### Pages and Frames
A Browser context can have multiple pages. A [`Page`](../api.md#class-page) A Browser context can have multiple pages. A [`Page`](../api.md#class-page)
refers to a single tab within a browser context. A page can be used to navigate refers to a single tab or a popup window within a browser context. A page can be used to navigate
to URLs and then interact with elements. to URLs and then interact with elements.
```js ```js
@ -83,7 +79,7 @@ const frame = page.frame('frame-name');
await frame.fill('#username-input'); await frame.fill('#username-input');
``` ```
#### Single Page Scenarios ### Single Page Scenarios
For scenarios involving just one page, it is possible to create a new page For scenarios involving just one page, it is possible to create a new page
without explicitly creating a browser context through a convenience API. This without explicitly creating a browser context through a convenience API. This
@ -97,22 +93,37 @@ const page = await browser.newPage();
## Execution contexts ## Execution contexts
The Playwright API can be used in a JavaScript file running in a Node.js Playwright scripts run in your Node.js environment. You page scripts run in the page environment. Those environments don't intersect, they are running in different virtual machines in different processes and potentially on different computers.
environment. When a browser instance is created, Playwright creates a
bi-directional communication channel between the Node.js script and the browser.
With this channel in place, Playwright can also execute JavaScript code in the IMAGE PLACEHOLDER
context of the web page. This enables another execution context for automation
scripts. Playwright exposes APIs to execute the JavaScript code in the context of the web page and allows calling back into the Node.js environment from your web page.
### Using the Evaluate API ### Using the Evaluate API
The [`page.evaluate`](https://github.com/microsoft/playwright/blob/master/docs/api.md#pageevaluatepagefunction-arg) API can run a JavaScript function in the context The [`page.evaluate`](https://github.com/microsoft/playwright/blob/master/docs/api.md#pageevaluatepagefunction-arg) API can run a JavaScript function in the context
of the web page and bring results back to the Node.js environment. Globals like of the web page and bring results back to the Node.js environment. Globals like
`window` and `document` can be used in `evaluate`. `window` and `document` along with the web page runtime can be used in `evaluate`.
Right:
```js ```js
const result = await page.evaluate(() => { const data = { text: 'some data', value: 1 };
return { width: window.innerWidth, height: window.innerHeight }; // Pass |data| as a parameter.
}) const result = await page.evaluate(data => {
window.myApp.use(data);
}, data);
``` ```
Wrong:
```js
const data = { text: 'some data', value: 1 };
const result = await page.evaluate(() => {
// There is no |data| in the web page.
window.myApp.use(data);
});
```
Evaluation parameters are serialized and sent into your web page over the wire.
You can pass primitive types, JSON-alike objects and remote object handles received from the page.