Awesome
rewire
Easy monkey-patching for node.js unit tests
rewire adds a special setter and getter to modules so you can modify their behaviour for better unit testing. You may
- inject mocks for other modules or globals like
process
- inspect private variables
- override variables within the module.
Please note: The current version of rewire is only compatible with CommonJS modules. See Limitations.
<br>Installation
npm install rewire
Introduction
Imagine you want to test this module:
// lib/myModule.js
// With rewire you can change all these variables
var fs = require("fs"),
path = "/somewhere/on/the/disk";
function readSomethingFromFileSystem(cb) {
console.log("Reading from file system ...");
fs.readFile(path, "utf8", cb);
}
exports.readSomethingFromFileSystem = readSomethingFromFileSystem;
Now within your test module:
// test/myModule.test.js
var rewire = require("rewire");
var myModule = rewire("../lib/myModule.js");
rewire acts exactly like require. With just one difference: Your module will now export a special setter and getter for private variables.
myModule.__set__("path", "/dev/null");
myModule.__get__("path"); // = '/dev/null'
This allows you to mock everything in the top-level scope of the module, like the fs module for example. Just pass the variable name as first parameter and your mock as second.
var fsMock = {
readFile: function (path, encoding, cb) {
expect(path).to.equal("/somewhere/on/the/disk");
cb(null, "Success!");
}
};
myModule.__set__("fs", fsMock);
myModule.readSomethingFromFileSystem(function (err, data) {
console.log(data); // = Success!
});
You can also set multiple variables with one call.
myModule.__set__({
fs: fsMock,
path: "/dev/null"
});
You may also override globals. These changes are only within the module, so you don't have to be concerned that other modules are influenced by your mock.
myModule.__set__({
console: {
log: function () { /* be quiet */ }
},
process: {
argv: ["testArg1", "testArg2"]
}
});
__set__
returns a function which reverts the changes introduced by this particular __set__
call
var revert = myModule.__set__("port", 3000);
// port is now 3000
revert();
// port is now the previous value
For your convenience you can also use the __with__
method which reverts the given changes after it finished.
myModule.__with__({
port: 3000
})(function () {
// within this function port is 3000
});
// now port is the previous value again
The __with__
method is also aware of promises. If a thenable is returned all changes stay until the promise has either been resolved or rejected.
myModule.__with__({
port: 3000
})(function () {
return new Promise(...);
}).then(function () {
// now port is the previous value again
});
// port is still 3000 here because the promise hasn't been resolved yet
<br />
Limitations
Babel's ES module emulation<br> During the transpilation step from ESM to CJS modules, Babel renames internal variables. Rewire will not work in these cases (see #62). Other Babel transforms, however, should be fine. Another solution might be switching to babel-plugin-rewire.
Variables inside functions<br> Variables inside functions can not be changed by rewire. This is constrained by the language.
// myModule.js
(function () {
// Can't be changed by rewire
var someVariable;
})()
Modules that export primitives<br>
rewire is not able to attach the __set__
- and __get__
-method if your module is just exporting a primitive. Rewiring does not work in this case.
// Will throw an error if it's loaded with rewire()
module.exports = 2;
Globals with invalid variable names<br>
rewire imports global variables into the local scope by prepending a list of var
declarations:
var someGlobalVar = global.someGlobalVar;
If someGlobalVar
is not a valid variable name, rewire just ignores it. In this case you're not able to override the global variable locally.
Special globals<br>
Please be aware that you can't rewire eval()
or the global object itself.
API
rewire(filename: String): rewiredModule
Returns a rewired version of the module found at filename
. Use rewire()
exactly like require()
.
rewiredModule.__set__(name: String, value: *): Function
Sets the internal variable name
to the given value
. Returns a function which can be called to revert the change.
rewiredModule.__set__(obj: Object): Function
Takes all enumerable keys of obj
as variable names and sets the values respectively. Returns a function which can be called to revert the change.
rewiredModule.__get__(name: String): *
Returns the private variable with the given name
.
rewiredModule.__with__(obj: Object): Function<callback: Function>
Returns a function which - when being called - sets obj
, executes the given callback
and reverts obj
. If callback
returns a promise, obj
is only reverted after the promise has been resolved or rejected. For your convenience the returned function passes the received promise through.
Caveats
Difference to require()<br> Every call of rewire() executes the module again and returns a fresh instance.
rewire("./myModule.js") === rewire("./myModule.js"); // = false
This can especially be a problem if the module is not idempotent like mongoose models.
Globals are imported into the module's scope at the time of rewiring<br>
Since rewire imports all gobals into the module's scope at the time of rewiring, property changes on the global
object after that are not recognized anymore. This is a problem when using sinon's fake timers after you've called rewire()
.
Dot notation<br>
Although it is possible to use dot notation when calling __set__
, it is strongly discouraged in most cases. For instance, writing myModule.__set__("console.log", fn)
is effectively the same as just writing console.log = fn
. It would be better to write:
myModule.__set__("console", {
log: function () {}
});
This replaces console
just inside myModule
. That is, because rewire is using eval()
to turn the key expression into an assignment. Hence, calling myModule.__set__("console.log", fn)
modifies the log
function on the global console
object.
webpack
See rewire-webpack
<br />License
MIT