Wraps the top level functions of a Javascript object to allow before/after intercepts. Supports parameter substitutions via value or function.
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README.md

NoWrap v0.3.5

Wraps the top level functions of a Javascript object to allow before/after intercepts. Supports parameter substitutions via value or function.

By K Cartlidge.

Licence

MIT Licence (very permissive). See the GitHub licence summary bullet points here.

A copy of the licence is within the package source.

Current Status

  • Tested and working.
  • Includes optional parameter substitutions.

What Problem does it Solve?

It allows you to intercept calls to functions (on your own objects/modules or core/third party ones) and register handlers which get called before and/or after the intercepted one.

The real thing behaves as normal. Your before hook is told all the original parameters. Your after hook is told the result.

Installation

It's an npm module:

npm install nowrap

Usage:

Require nowrap and the thing you want to wrap:

var wrap = require('nowrap');

// Load a module to use as an example.
var path = require('path');

Create either/both your before and after hooks:

// Create a function to be called before a wrapped one.
var beforeFunc = function (name, args) {
	console.log("Actually calling '" + name + "' with ", args);
};

// Create a function to be called after a wrapped one.
var afterFunc = function (name, result) {
	console.log("The call returned:", result);
};

Create any substitutions required for parameters. Using these, you can dynamically alter what the function is given and so influence the result:

// The path goes uppercase.
// The ext becomes '.ORIGINAL'.
var substitutions = {
	basename: {
		path: function(original) {
			return original.toUpperCase();
		},
		ext: '.ORIGINAL'
	}
};

This is an object whose top level name should be the name of the function for which the substitution occurs. Within that, the next nested name is the parameter name and the that contains the value to be fed to the parameter.

If the value is anything other than a function it is fed in unchanged. If it is a function it is called with the value that would normally be passed at this time, and is expected to then return a replacement for passing in instead.

In the example above, when basename.path('file.original','.ext') is called then 'file.original' is changed to 'FILE.ORIGINAL' via the function and '.ext' is changed to '.ORIGINAL' via the value substitution.

Wrap the thing and use it:

// Wrap the main functions and re-run.
wrap(path, beforeFunc, afterFunc, substitutions);
console.log("Appears to be calling 'basename' with ('/path/file.original', '.ext').");
var result = path.basename('/path/file.original', '.ext');

The output is similar to:

Appears to be calling 'basename' with ('/path/file.original', '.ext').
Actually calling 'basename' with  [ path: '/PATH/FILE.ORIGINAL', ext: '.ORIGINAL' ]
The call returned: FILE

The args passed to the before function, being an associative array, can also be accessed as so:

var beforeFunc = function(name, args) {
	console.log(args.path);
};

Test Coverage

You can run the tests using npm:

npm test

Examples

You can try it live on the npm page. This will run the tonic-example.js file (which is included).