rewire
======
**Easy monkey-patching for node.js unit tests**
[](https://www.npmjs.com/package/rewire)
[](https://www.npmjs.com/package/rewire)
[](https://coveralls.io/r/jhnns/rewire?branch=master)
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](https://github.com/jhnns/rewire#limitations).
Installation
------------
`npm install rewire`
Introduction
------------
Imagine you want to test this module:
```javascript
// 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:
```javascript
// 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.
```javascript
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.
```javascript
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.
```javascript
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.
```javascript
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
```javascript
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.
```javascript
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.
```javascript
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
```
Limitations
-----------
**Babel's ES module emulation**
During the transpilation step from ESM to CJS modules, Babel renames internal variables. Rewire will not work in these cases (see [#62](https://github.com/jhnns/rewire/issues/62)). Other Babel transforms, however, should be fine. Another solution might be switching to [babel-plugin-rewire](https://github.com/speedskater/babel-plugin-rewire).
**Variables inside functions**
Variables inside functions can not be changed by rewire. This is constrained by the language.
```javascript
// myModule.js
(function () {
// Can't be changed by rewire
var someVariable;
})()
```
**Modules that export primitives**
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.
```javascript
// Will throw an error if it's loaded with rewire()
module.exports = 2;
```
**Globals with invalid variable names**
rewire imports global variables into the local scope by prepending a list of `var` declarations:
```javascript
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**
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()**
Every call of rewire() executes the module again and returns a fresh instance.
```javascript
rewire("./myModule.js") === rewire("./myModule.js"); // = false
```
This can especially be a problem if the module is not idempotent [like mongoose models](https://github.com/jhnns/rewire/issues/27).
**Globals are imported into the module's scope at the time of rewiring**
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()`](http://stackoverflow.com/questions/34885024/when-using-rewire-and-sinon-faketimer-order-matters/36025128).
**Dot notation**
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:
```javascript
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](https://github.com/jhnns/rewire-webpack)
## License
MIT