How to proxy to many different targets using http-proxy-middleware?

Question

In the http-proxy-middleware library, the documentation states that you can use the target option to specify where you would like to proxy the request. However, they also allow you to use the router option to specify a function that will be used to functionally resolve the target at runtime.

Docs: https://www.npmjs.com/package/http-proxy-middleware

I am using TypeScript and if I look at the declaration file for the proxy I can see this:

You can see here that both router and target are nullable. My assumption would be that if you use one, the other can be omitted, but you always need at least 1.

However, if I use the router property like this:

app.use("/pipe", proxy({
    changeOrigin: true,
    router: (req: IIncomingMessageWithCookies) => {
        return "https://www.google.com";
    }
}));

And omit the target, then at runtime I get this error:

> node ./dist/app.js

C:\SkyKick\SkyKick.SEWR\src\node_modules\http-proxy-middleware\lib\config-factory.js:43
    throw new Error(ERRORS.ERR_CONFIG_FACTORY_TARGET_MISSING)
    ^

Error: [HPM] Missing "target" option. Example: {target: "http://www.example.org"}
    at Object.createConfig (C:\SkyKick\SkyKick.SEWR\src\node_modules\http-proxy-middleware\lib\config-factory.js:43:11)
    at new HttpProxyMiddleware (C:\SkyKick\SkyKick.SEWR\src\node_modules\http-proxy-middleware\lib\index.js:17:30)
    at module.exports (C:\SkyKick\SkyKick.SEWR\src\node_modules\http-proxy-middleware\index.js:4:10)
    at Object.<anonymous> (C:\SkyKick\SkyKick.SEWR\src\dist\app.js:8:18)
    at Module._compile (internal/modules/cjs/loader.js:689:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
    at Module.load (internal/modules/cjs/loader.js:599:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
    at Function.Module._load (internal/modules/cjs/loader.js:530:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:742:12)

I realized that I can place pretty much anything into target and it will run just fine and my router function is the thing that actually defines the target proxy. Is this just a bug with the library or am I misunderstanding what these 2 options are used for?

Curfew

Answer 1

Include both the target and the router property. The router property is there to re-target option.target for specific requests.

import express = require('express');
import proxy = require('http-proxy-middleware');

const app = express();

app.use('/api', proxy({
    target: 'http://www.github.com',
    changeOrigin: true,
    router: function (req: IncomingMessage) {
        return 'http://www.stackoverflow.com'
    }
}));

app.listen(3000);

The target property is optional in the Config type because it is allowed to be empty when we use the shorthand like this:

app.use('/api', proxy('http://www.github.com',
    {
        changeOrigin: true,
        router: function (req: IncomingMessage) {
            return 'http://www.stackoverflow.com'
        }
    }
));

Schulz

Ok that totally makes sense regarding the nullable properties, however I still don't think I understand what it means to force the developer to re-target like this rather than just having many different ways to define a single property. In my scenario I am wanting to take some arbitrary formatted domain and URL schema and map these origins to many different targets. In that case, I don't have any 1 target to use, do I? – Curfew

I took a look at the source code and do see that router basically just overrides the original options.target property like I imagined, however it still feels weird being required to add a target in this case. I feel like the validation logic should check for the existence of at least one, regardless of implementation. – Curfew

Regarding the mapping that you are wanting to do, have you looked at setting the router value to an object instead of to a function? – Schulz

I can see what you mean about throwing an error when the target is empty and the router has a value. At first blush, it seems lazy on the part of the original developers. I appreciate, though, that there might be some deeper reason that will reveal itself on further investigation. And, if there is no deeper reason, setting target to something like http://required-target.why is a reasonable and inexpensive workaround. – Schulz

I don't think I can set it to an object because that would mean there was a finite set of mappings that would take place, when in fact I need the mappings to be completely conventional and scale to any number all based off the schema of the incoming request URI. With that said, simply putting some "garbage" into the target property seems to work just fine. I just wanted to confirm that I was understanding how it worked. It seemed farfetched that this was an issue with the library itself seeing how its so widely adopted. Thanks! – Curfew

What happens when one wants to get function from another js file? What would be the syntax? – Cruikshank