Puppeteer: Custom object serialization not possible. Use plain objects instead.

The BiDi serializer cannot encode a custom object instance.

Error message

Custom object serialization not possible. Use plain objects instead.

How this error happens

WebDriver BiDi cannot serialize an arbitrary class instance as a function argument. Its methods and prototype do not form a plain data payload.

The snippets use a WebDriver BiDi page named page.

class Settings {
    constructor() {
        this.theme = "dark";
    }
}

await page.evaluate(
    (settings) => settings.theme,
    new Settings()
);

How to fix

Pass a plain object with the values the page needs.

const settings = {
    theme: "dark",
};

const theme = await page.evaluate(
    (value) => value.theme,
    settings
);

console.log(theme);

Things to keep in mind

Do not assume every value can be fixed with JSON.stringify. Functions, symbols, cycles and special data types need an intentional representation.

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