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Asserts your function invariants

build status # annotate - Annotate your JavaScript function definitions

annotate allows you to ... guess what ... annotate your functions. For instance you could document invariants of your function. Or attach a description to it. It is possible to access this data later on.

This metadata can be used by tools such as annofuzz in order to generate tests. In addition you can access the metadata via REPL.

The usage is quite simple as the following example illustrates:

// let's define some function to annotate 
function add(a, b) {
    return a + b;
// type checkers from annois ( 
var addNumbers = annotate('addNumbers', 'Adds numbers').
    on(is.number, is.number, add);
var addStrings = annotate('addStrings', 'Adds strings').
    on(is.string, is.string, add);
// you can assert invariants too 
var addPositive = annotate('addPositive', 'Adds positive').
    on(isPositive, isPositive, add).
    satisfies(isPositive); // postcondition 
// it is possible to chain guards 
var fib = annotate('fib', 'Calculates Fibonacci numbers').
    on(0, 0).on(1, 1).
    on(is.number, function(n) {
        return fib(- 1) + fib(- 2);
// invariants may depend on each other 
var clamp = annotate('clamp', 'Clamps given number between given bounds').
    on(is.number, is.number, function(a, args) {
        return is.number(a) && args[1] <= a;
    }, function(a, min, max) {
        return Math.max(Math.min(a, max), min);
// furthermore it is possible to pass a variable amount of args 
var min = annotate('min', 'Returns minimum of the given numbers').
    on([is.number], Math.min);
function isPositive(a) {
    return a >= 0;

The annotate function will create a new function that contains the metadata as properties _name, _doc, _preconditions and _postconditions. In case some pre- or postcondition doesn't pass it won't return and gives a warning instead.

Related Projects

  • suite.js - Constructs tests based on invariant data (fuzzing)
  • funkit - Collection of utilities tested using annotate.js and suite.js



annotate is available under MIT. See LICENSE for more details.