Add support for conditional loading of middleware

[gustavnikolaj]

I have a need to be able to install middleware based on the value of configuration.

It could be done with express-noop like this:

app.use(condition ? middleware : require('express-noop')());

That is doing it's job, but it is wasteful, as there is no need to register any middleware if the condition is false. It's only being evaluated up front any way.

It could also be done by using an if sentence.

var app = express();

if (condition) {
    app.use(middleware);
}

But it is annoying when you are nesting express applications to build structures:

var app = express();
var app2 = express();

if (condition) {
    app2.use(middleware);
}

app2.use(bar)

app.use(foo)
   .use(app2);

With this PR you could use .useif() instead of use and make it all much more convenient and readable:

var app = express();

app
  .use(foo)
  .use(express()
    .useif(condition, middleware)
    .use(bar));

I hope that I made my motivation and use-case clear, and that the test coverage and code style lives up to your expectations. If there is anything that I can improve, I'll be more than happy to change it.

[gustavnikolaj]

I have been using this technique already and have grown quite fond of it.

For reference, this is the code I have used to monkey patch it into express:

require('express/lib/application').useif = function (condition) {
    if (condition) {
        var args = Array.prototype.slice.call(arguments, 1);
        return this.use.apply(this, args);
    }
    return this;
};

[Twipped]

This is the exact use case that I created pitstop for. It's also what Issue #2241 was hoping to address.

[gustavnikolaj]

@ChiperSoft, This is supposed to only have affect when initially registering the middleware. The use of next(...) described in #2241 seems more like a runtime thing to me. I might misunderstand it...

I see how pitstop can solve the same problem, but it has a wider scope, as I guess that the function that you can pass to pit.condition() will be evaluated at runtime too. My .useif() suggestion is like pit.condition() but static...

[Twipped]

Oh, this is just about deciding if the middleware should even be added to that stack at startup?

Yeah, no, use an if statement. 👎

[gustavnikolaj]

I extended the patch than I'm using and split it out into it's own module. https://github.com/gustavnikolaj/express-use-if

[dougwilson]

Closing, mostly because we feel it should be solved in user-land.