Simplest Javascript interception library on the planet!

Usage no npm install needed!

<script type="module">
  import easyIntercept from 'https://cdn.skypack.dev/easy-intercept';



Simplest Javascript interception framework on the planet.

easy-intercept is a function interception library, nothing more nor less. It's commonly used for unit testing with other utilities like mocha and proxyquire. However, it can be used for any other intercepting scenarios you fancy.


npm install easy-intercept

Create an intercepted function.

var intercept = require('easy-intercept');
var f = intercept();

Configure a return value.


Throw an error.


Configure a return value on specific call.


There are some helpers to configure most common calls.


Throw an error on a specific call.


Configure a return value for a call with matching arguments.

f.withArgs('a', 'b').returns(42);

Throw an error for matching arguments.

f.withArgs('a', 'b').throws('doh');

Access calls received with specific arguments. received method returns an array of callInfo objects containing the call information.

var calls = f.received('a', 'b');
var numberOfCalls = calls.length;

Verify if a call with any argument is made.

f.receivedAny('a', 'b');

Use proxy._ match any argument.

f.received(intercept._, 'b');

Receive an array of callInfo containing all calls made to an intercepted function.


Passing an existing functions to intercept will create a new function that can be manipulated as shown in preceding examples. At runtime, if a specific behavior cannot be found, the original function is executed.

function add(a, b) {
  return a + b;

var p = intercept(print);
p(1, 1); // 42
p(1, 1); // 2

p.withArgs(10, 5).returns(42);
p(10, 5); // 42
p(1, 1); // 2;

p.withArgs(2, 2).throws('doh');
p(2, 2); // throws
p(1, 1); // 2

Passing an object to intercept will create a new object mirroring all functions available in the original object. Behavior of these functions is identical to the behavior described above.

function Foo() {
  this.add = function(a, b) {
    return a + b;

var f = new Foo();
var i = intercept(f);

i.add(1, 1); // 42
i.add(2, 2); // 4