keep-or-skip

keep-or-skip

Dynamically execute or skip express middlewares

This wrapper allows you to choose one or more middlewares to execute based on a certain condition. The aforementioned condition could be, for instance, a specific request value.

Installation

You can choose one of the following methods:

First method

In your package.json add the following item to get the latest available version:

"keep-or-skip": "*"

or, if you need a specific version, just add an item like the following, specifying the version number:

"keep-or-skip": "1.0.1"

then launch this command:

npm install

Second method

Just launch this command:

npm install keep-or-skip --save

Type signature

keepOrSkip(middlewares, predicate[, debug])

• middlewares <Function> | <Function[]> A middleware or an array of middlewares to handle dynamically.
• predicate <Function> A function that returns a boolean value, by which, one or more middlewares will be executed or skipped. This function takes as input two optional parameters, the request and the response objects. If the predicate doesn't return a boolean value, the middleware/middlewares contained in the middlewares parameter will be skipped.
• debug <Boolean> An optional boolean parameter to enable a warning log which notifies that the predicate parameter doesn't return a boolean value. Default: false.
• returns An array of middlewares.

If the parameters' type does not match with those required, an error will be thrown. In pariticular, the error will be an instance of KeepOrSkipError.

Examples

Consider the following example(all the middlewares defined have demonstration purposes only):

const express = require('express')
const keepOrSkip = require('keep-or-skip')

function m1(req, res, next) {
    if (!req.middlewares) {
        req.middlewares = []
    }
    req.middlewares.push('executed m1')
    next()
}

function m2(req, res, next) {
    if (!req.middlewares) {
        req.middlewares = []
    }
    req.middlewares.push('executed m2')
    next()
}

function respond(req, res, next) {
    res.status(200).json({
        middlewares: req.middlewares
    })
}

function setRandomValue(req, res, next) {
    var max = 5
    var min = -5
    req.random = Math.random() * (max - min) + min
    next()
}

app.get('/random',
    setRandomValue,
    keepOrSkip([m2, m1, m2], (req, res) => {
        if (req.random < 0) {
            // Execute [m2, m1, m2]
            return true
        }
        // Skip [m2, m1, m2]
        return false
    }),
    keepOrSkip([m1, m2], (req, res) => {
        if (req.random >= 0) {
            // Execute [m1, m2]
            return true
        }
        // Skip [m1, m2]
        return false
    }),
    respond
)

app.listen(3000)

http://localhost:3000/random will produce the following result if the random value is lower than 0:

{
    "middlewares": [
        "executed m2",
        "executed m1",
        "executed m2"
    ]
}

otherwise it will produce:

{
    "middlewares": [
        "executed m1",
        "executed m2"
    ]
}

Debug

In the case the predicate parameter doesn't return a boolean value, the middleware/middlewares contained in the middlewares parameter will be skipped. In this situation, it's possible to log a warning by using the debug mode.

It's possible to activate the debug globally, directly on the imported module:

const keepOrSkip = require('keep-or-skip')
keepOrSkip.debug(true)

the above code will set the debug mode to true every time keepOrSkip will be used, unless otherwise specified at the time of use as shown in the following example:

const keepOrSkip = require('keep-or-skip')

// Set debug mode globally.
keepOrSkip.debug(true)

/**
 * Specifying the debug parameter at the time of use will overwrite the global debug variable.
 */

keepOrSkip(myMiddlewares, myPredicate, false) // Debug OFF.

keepOrSkip(myMiddlewares, myPredicate) // Debug ON because of the global debug variable.