hubcap eval

Evaluate a JavaScript expression in the page context and return the result.

When to use

Evaluate a JavaScript expression in the page context. Returns the result value and type. If the expression returns a Promise, it is automatically awaited. Use run to execute a JS file instead of an inline expression. Use evalframe to evaluate inside a specific frame.

Usage

hubcap eval <expression>

Arguments

Argument	Type	Required	Description
`expression`	string	Yes	JavaScript expression to evaluate in the page context

Flags

None.

Output

Field	Type	Description
`value`	any	The returned value from the expression
`type`	string	JavaScript type of the result (`string`, `number`, `boolean`, `object`, `undefined`)

{"value":42,"type":"number"}

Errors

Condition	Exit code	Stderr
JavaScript evaluation error	1	`error: JS exception: <message>`
Chrome not connected	2	`error: connecting to Chrome: ...`
Timeout	3	`error: timeout`

Examples

Get the page title:

hubcap eval 'document.title'

Count elements matching a selector:

hubcap eval 'document.querySelectorAll("a").length'

Return a JSON object:

hubcap eval '({width: window.innerWidth, height: window.innerHeight})'

Await an async operation:

hubcap eval 'fetch("/api/data").then(r => r.json())'

Chain with jq to extract the value and use in a shell pipeline:

hubcap eval 'document.querySelectorAll("li").length' | jq -r '.value'