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docs: add documentation for selector engines (#752)
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# Selector engines | ||
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Playwright supports multiple selector engines used to query elements in the web page. | ||
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Selector can be used to obtain `ElementHandle` (see [page.$()](api.md#pageselector) for example) or shortcut element operations to avoid intermediate handle (see [page.click()](api.md#pageclickselector-options) for example). | ||
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## Selector syntax | ||
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Selector is a string that consists of one or more clauses separated by `>>` token, e.g. `clause1 >> clause2 >> clause3`. When multiple clauses are present, next one is queried relative to the previous one's result. | ||
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Each clause contains a selector engine name and selector body, e.g. `engine=body`. Here `engine` is one of the supported engines (e.g. `css` or a custom one). Selector `body` follows the format of the particular engine, e.g. for `css` engine it should be a [css selector](https://developer.mozilla.org/en/docs/Web/CSS/CSS_Selectors). Body format is assumed to ignore leading and trailing whitespaces, so that extra whitespace can be added for readability. If selector engine needs to include `>>` in the body, it should be escaped inside a string to not be confused with clause separator, e.g. `text="some >> text"`. | ||
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For example, | ||
``` | ||
css=article >> css=.bar > .baz >> css=span[attr=value] | ||
``` | ||
is equivalent to | ||
```js | ||
document | ||
.querySelector('article') | ||
.querySelector('.bar > .baz') | ||
.querySelector('span[attr=value]') | ||
``` | ||
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For convenience, selectors in the wrong format are heuristically converted to the right format: | ||
- selector starting with `//` is assumed to be `xpath=selector`; | ||
- selector starting with `"` is assumed to be `text=selector`; | ||
- otherwise selector is assumed to be `css=selector`. | ||
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## Examples | ||
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```js | ||
// queries 'div' css selector | ||
const handle = await page.$('css=div'); | ||
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// queries '//html/body/div' xpath selector | ||
const handle = await page.$('xpath=//html/body/div'); | ||
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// queries '"foo"' zs selector | ||
const handle = await page.$('zs="foo"'); | ||
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// queries 'span' css selector inside the result of '//html/body/div' xpath selector | ||
const handle = await page.$('xpath=//html/body/div >> css=span'); | ||
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// converted to 'css=div' | ||
const handle = await page.$('div'); | ||
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// converted to 'xpath=//html/body/div' | ||
const handle = await page.$('//html/body/div'); | ||
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// converted to 'text="foo"' | ||
const handle = await page.$('"foo"'); | ||
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// queries 'span' css selector inside the div handle | ||
const handle = await divHandle.$('css=span'); | ||
``` | ||
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## Built-in selector engines | ||
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### css | ||
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CSS engine is equivalent to [`Document.querySelector`](https://developer.mozilla.org/en/docs/Web/API/Document/querySelector). Example: `css=.article > span:nth-child(2) li`. | ||
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> **NOTE** Malformed selector not starting with `//` nor with `#` is automatically transformed to css selector. For example, Playwright converts `page.$('span > button')` to `page.$('css=span > button')`. Selectors starting with `#` are converted to [text](#text). Selectors starting with `//` are converted to [xpath](#xpath). | ||
### xpath | ||
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XPath engine is equivalent to [`Document.evaluate`](https://developer.mozilla.org/en/docs/Web/API/Document/evaluate). Example: `xpath=//html/body`. | ||
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> **NOTE** Malformed selector starting with `//` is automatically transformed to xpath selector. For example, Playwright converts `page.$('//html/body')` to `page.$('xpath=//html/body')`. | ||
### text | ||
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Text engine finds an element that contains a text node with passed text. Example: `text=Login`. | ||
- By default, the match is case-insensitive, and ignores leading/trailing whitespace. This means `text= Login` matches `<button>loGIN </button>`. | ||
- Text body can be escaped with double quotes for precise matching, insisting on specific whitespace and case. This means `text="Login "` will only match `<button>Login </button>` with exactly one space after "Login". | ||
- Text body can also be a JavaScript-like regex wrapped in `/` symbols. This means `text=/^\\s*Login$/i` will match `<button> loGIN</button>` with any number of spaces before "Login" and no spaces after. | ||
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> **NOTE** Malformed selector starting with `"` is automatically transformed to text selector. For example, Playwright converts `page.click('"Login"')` to `page.click('text="Login"')`. | ||
### id, data-testid, data-test-id, data-test | ||
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Id engines are selecting based on the corresponding atrribute value. For example: `data-test-id=foo` is equivalent to `querySelector('*[data-test-id=foo]')`. | ||
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### zs | ||
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ZSelector is an experimental engine that tries to make selectors survive future refactorings. Example: `zs=div ~ "Login"`. | ||
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TODO: write more. | ||
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## Custom selector engines | ||
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Playwright supports custom selector engines, registered with [selectors.register(engineFunction[, ...args])](api.md#selectorsregisterenginefunction-args). | ||
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Selector engine should have the following properties: | ||
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- `name` Selector name used in selector strings. | ||
- `create` Function to create a relative selector from `root` (root is either a `Document`, `ShadowRoot` or `Element`) to a `target` element. | ||
- `query` Function to query first element matching `selector` relative to the `root`. | ||
- `queryAll` Function to query all elements matching `selector` relative to the `root`. | ||
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An example of registering selector engine that queries elements based on a tag name: | ||
```js | ||
// Must be a function that evaluates to a selector engine instance. | ||
const createTagNameEngine = () => ({ | ||
// Selectors will be prefixed with "tag=". | ||
name: 'tag', | ||
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// Creates a selector that matches given target when queried at the root. | ||
// Can return undefined if unable to create one. | ||
create(root, target) { | ||
return root.querySelector(target.tagName) === target ? target.tagName : undefined; | ||
}, | ||
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// Returns the first element matching given selector in the root's subtree. | ||
query(root, selector) { | ||
return root.querySelector(selector); | ||
}, | ||
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// Returns all elements matching given selector in the root's subtree. | ||
queryAll(root, selector) { | ||
return Array.from(root.querySelectorAll(selector)); | ||
} | ||
}); | ||
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// Register the engine. | ||
await selectors.register(createTagNameEngine); | ||
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// Now we can use 'tag=' selectors. | ||
const button = await page.$('tag=button'); | ||
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// We can combine it with other selector engines. | ||
await page.click('tag=div >> text="Click me"'); | ||
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// We can use it in any methods supporting selectors. | ||
const buttonCount = await page.$$eval('tag=button', buttons => buttons.length); | ||
``` |