ferllop / preconditions

A library to assert preconditions

Almighty Simple Preconditions

A simple library to be able to assert preconditions in our javascript methods or functions.

No matchers, just boolean operations.

Activation

You have to activate preconditions explicitly setting a ENABLE_PRECONDITIONS to true as an environment variable. You can do it in two ways:

1. Enable when executing the command. For example:

$ ENABLE_PRECONDITIONS=true node index.js 

or in npm script:

"scripts" : {
    start: "ENABLE_PRECONDITIONS=true node index.js"
}

2. Enable in your environment running:

export ENABLE_PRECONDITIONS=true

and then you can run your commands as usually, like:

$ node index.js

For having this last option working after rebooting your machine, remember to put it in your .bashrc (or similar if you use another shell)

Usage

This will throw a PreconditionError because name is less than two strings:

5    function greeting(name) {
6        precondition(typeof name === "string" && name.length > 2)
7        return 'Hello, ' + name
8    }
9    
10    greeting('Pi')

And the error.message will be like this:

Precondition "typeof name === "string" && name.length > 2", 
written on line 6 of file "/some-project/example.js", 
was offended by call "greeting('Pi')", 
written on line 10 of file "/some-project/example.js".

EXAMPLE

You can run a more complete example running:

$ npm run example