How to use Fetch in JavaScript (V8go, Otto) in Golang?

Question

I am trying to run JavaScript inside of a Golang function, and use fetch to fetch JSON via APIs in the Javascript context.

I tried it in Otto with the following code:

import "github.com/robertkrimen/otto"

    vm := otto.New()
    vm.Run(`
    function tryfunc() {
        console.log("Started");
        fetch('https://www.example.com/APIendpoint');
        console.log("Success");
    }
    tryfunc();
    `)

Otto is very simple to use, but looks like Otto is an event bus and does not manage fetch.

I am now trying with v8go with the following code:

import v8 "rogchap.com/v8go"

ctx := v8.NewContext()
ctx.RunScript(`fetch("https://www.example.com/APIendpoint"), ""`)

but it requires another argument. The documentation is very unclear and is difficult to understand how to run even the simplest JS script.

Can someone help?

`fetch` is a bit more complicated than what you seem to assume in your code. The main issue will be, I think, that `fetch` is asynchronous. See [Using the Fetch API](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch). — Ouroborus, Jul 05 '22 at 16:38
@Ouroborus I did write ```fetch``` without ```await``` to simplify the question. Anyhow, just using it as in JS would not work. — Filippo, Jul 06 '22 at 09:51

Wagner Riffel · Accepted Answer · 2022-07-05T19:34:28.283

1

Otto and v8 implements ECMAScript and as of current 13.0 version doesn't require a builtin function fetch (See: https://262.ecma-international.org/13.0/#sec-function-properties-of-the-global-object), this function is implemented by web browsers.

I believe the second argument is simply a file name to prefix debug output (eg: foo.js:14: undefined bar), that's very usual in interpreters.

I suspect your script did ran, if you print out the output from RunScript (you're ignore both return values, a JavaScript value and an error), that should have an "undefined function fetch" and value nil. FYI there is an example for implementing fetch in v8go documentation.

edited Jul 05 '22 at 19:34

answered Jul 05 '22 at 17:01

Wagner Riffel

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Thank you for your comment. My aim was in fact to run frontend code (that runs on browsers) in the backend. I would not try to use JS in Golang otherwise. In the end it was simpler just to fetch in Golang. – Filippo Jul 06 '22 at 09:55

How to use Fetch in JavaScript (V8go, Otto) in Golang?

1 Answers1