stand-in

Method replacement and testing utility

Description

Often when unit testing, it is helpful to capture or replace function calls with testing code. stand-in provides a quick and easy way to replace object methods with your own. It also provides a method to restore the original method after testing is complete.

Usage

var standin = require('stand-in');
var assert = require('assert');
var log = standin.replace(console, 'log', function (stand, value) {
  assert.strictEqual(value, 'test data', 'value should equal test data');
  stand.restore();
});

console.log('test data');

• replace(obj, path, fn [, options]) - replaces obj[path] with fn where:
  • obj - object that has the method to replace. Will be used at this pointer inside fn.
  • path - string path to the function to replace. Supports deep paths via "foo.bar.baz".
  • fn - function to replace obj[method] with. The first argument to this function will be a stand-in object. This is helpful if you don't want to create a holding variable.
  • options - an optional object supporting the following properties.
    • startOn - the invocation number to begin using the replacement function. Defaults to 0.
    • stopAfter - the final invocation number to use the replacement function. Once this number is reached, the stand-in object will restore() itself. Defaults to Infinity.

Returns a stand-in object:

• restore() - restores the original obj[method] to the previous function. Generally, this will restore the method back to the initial value.
• original - a handle to the original method in case you need to conditionally call it.
• invocations - the number of times the stand-in has been called.

replaceOnce() can be used to create a one time replacement. It is identical to replace(), except the stopAfter option will be set automatically to an appropriate value, depending on the value of startOn.

Note

replace tries to prevent users from completely losing a handle to the original method. For example, you will receive an AssertionError if you try to replace console.log twice without first restoreing the function first.