path: root/README.md

rewire
======
**Easy monkey-patching for node.js unit tests**

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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).

<br>

Installation
------------

`npm install rewire`

<br />

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
```

<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](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**<br>
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**<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.

```javascript
// 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:

```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**<br>
Please be aware that you can't rewire `eval()` or the global object itself.


<br />

API
---

### rewire(filename: String): rewiredModule

Returns a rewired version of the module found at `filename`. Use `rewire()` exactly like `require()`.

### rewiredModule.&#95;&#95;set&#95;&#95;(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.&#95;&#95;set&#95;&#95;(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.&#95;&#95;get&#95;&#95;(name: String): *

Returns the private variable with the given `name`.

### rewiredModule.&#95;&#95;with&#95;&#95;(obj: Object): Function&lt;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.

<br />

Caveats
-------

**Difference to require()**<br>
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**<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()`](http://stackoverflow.com/questions/34885024/when-using-rewire-and-sinon-faketimer-order-matters/36025128).

**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:

```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.

<br />

webpack
-------
See [rewire-webpack](https://github.com/jhnns/rewire-webpack)

<br />

CoffeeScript
------------

Good news to all caffeine-addicts: rewire works also with [Coffee-Script](http://coffeescript.org/). Note that in this case you need to install the `coffeescript` package.

<br />

## License

MIT