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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 .
Installation
npm install rewire
Introduction
Imagine you want to test this module:
Now within your test module:
rewire acts exactly like require. With just one difference: Your module will now export a special setter and getter for private variables.
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.
You can also set multiple variables with one call.
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.
__set__ returns a function which reverts the changes introduced by this particular __set__ call
For your convenience you can also use the __with__ method which reverts the given changes after it finished.
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.
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 ). Other Babel transforms, however, should be fine. Another solution might be switching to babel-plugin-rewire .
Variables inside functions Variables inside functions can not be changed by rewire. This is constrained by the language.
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.
Globals with invalid variable names rewire imports global variables into the local scope by prepending a list of var declarations:
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.
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.
Difference to require() Every call of rewire() executes the module again and returns a fresh instance.
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 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 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:
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.
See rewire-webpack
Package Sidebar
npm i rewire
Git github.com/jhnns/rewire
github.com/jhnns/rewire
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