javascript-runtime.md

Javascript Runtime

We can use Rod to evaluate random javascript code on the page. Such as use it to read or modify the HTML content of the page.

Eval on the page

For example use Page.Eval to set global value:

page.MustEval(`() => window.a = {name: 'jack'}`)

We can use the arguments of the js function to receive golang variables, here's how we get the key and data from golang and set it to the page's window object k:

key := "a"
data := map[string]string{"name": "jack"}
page.MustEval(`(k, val) => {
    window[k] = val
}`, key, data)

To get the returned value from Eval:

val := page.MustEval(`() => a`).Get("name").Str()
fmt.Println(val) // output: jack

Eval only accepts js function, code like below won't work:

page.MustEval(`a`) // will fail

Eval on an element

Element.Eval is similar with Page.Eval, but with the this object set to the current element. For example, we have a <button>Submit</button> on the page, we can read or modify the element with JS:

el := page.MustElement("button")
el.MustEval(`() => this.innerText = "Apply"`) // Modify the content
txt := el.MustEval(`() => this.innerText`).Str()
fmt.Println(txt) // output: Apply

Expose Go functions to the page

We can use Page.Expose to expose callback functions to the page. For example, here we expose a function to help the page to calculate md5 hash:

page.MustExpose("md5", func(g gson.JSON) (interface{}, error) {
    return md5.Sum([]byte(g.Str())), nil
})

Now the page can invoke this method on the window object:

hash := page.MustEval(`() => window.md5("test")`).Str()
fmt.Println(hash)

Here's another example to get button click event on the page:

page.MustExpose("myClick", func(v gson.JSON) (interface{}, error) {
    fmt.Println("Clicked")
    return nil, nil
})

Call the 'myClick' method when a button is clicked:

page.MustElement("button").MustEval(`() => this.onclick = myClick`)