Initializers

You can add initializers (aka constructors) to your stamps.
const Logger = stampit({
  init({level = 50}) {
    this.level = level
  }
})
​
const logger = Logger({ level: 42 })
​
logger.level === 42
Each initializer will be executed on object creation. (But the list of initializers is always deduplicated.)
const Server = stampit(Logger, {
  init({ port = process.env.PORT }) {
    this.port = port
  }
})
​
const server = Server({ port: 6666 })
​
server.level === 50
server.port === 6666
If you return anything from an initializer then it becomes the object instance.
const NullServer = Server.init(function () {
  return null
})
​
const server = NullServer()
​
server === null
Descriptor merging algorithm
The initializers are concatenated into a deduplicated array. As the result, the order of composition becomes the order of initializer execution.
const {init} = stampit
​
const Log1 = init(() => console.log(1))
const Log2 = init(() => console.log(2))
const Log3 = init(() => console.log(3))
​
const MultiLog = stampit(Log1, Log2, Log3)
​
MultiLog() // Will print three times:
// 1
// 2
// 3
​
// because there are 3 initializers
MultiLog.compose.initializers.length === 3
Stamps remove duplicate initializers.
const {init} = stampit
​
const func = () => console.log(1)
const Log1 = init(func)
const Log2 = init(func)
const Log3 = init(func)
​
const MultiLog = stampit(Log1, Log2, Log3)
​
MultiLog() // Will print only once:
// 1
​
// because there is only one initializer
MultiLog.compose.initializers.length === 1
Initializer arguments
NOTE
Parameter - a variable being passed to a function.
Argument - a variable received by a function.
You can pass multiple parameters to your stamp while creating an object. First parameter passed as is to all initializers. But the rest of the parameters are available inside the {args} property of the second initializer argument.
const MultiArg = stampit({
  init(arg1, { args }) {
    arg1 === 'foo'
    args[0] === arg1  // first argument of the initializer is passed from factory first parameter
    args[0] === 'foo'
    args[1] === 'BAR'
    args[2] === 'thing'
  }
})
​
MultiArg('foo', 'BAR', 'thing')
If there is no first parameter then an empty object is passed as the first argument.
const NoException = stampit({
  init({ iAmUndefined }) { // won't throw exception in this line
    iAmUndefined === undefined
    console.log(arguments[0]) // will print empty object: {}
  }
})
​
NoException(/* nothing here! */)
Every initializer second argument is always this object: { stamp, args, instance }. Where:
stamp is the stamp which was used to create this object. Useful to retrieve stamp's metadata (aka descriptor).
args the parameters passed to the stamp while creating the object.
instance the object instance itself. Always equals this context of the initializer.
const PrintMyArgs = stampit({
  init(_, { stamp, args, instance }) {
    console.log('Creating object from this stamp:', stamp)
    console.log('List of arguments:', args)
    console.log('Object instance:', instance)
​
    this === instance
  }
})
Other ways to add initializers
Exactly the same stamp can be created in few ways. Here they all are.
function myInitializer ({ level = 50 }) {
  this.level = level
}
​
const Logger = stampit({
  init: myInitializer
})
​
const Logger = stampit({
  init: [myInitializer]
})
​
const Logger = stampit({
  initializers: myInitializer
})
​
const Logger = stampit({
  initializers: [myInitializer]
})
​
const Logger = stampit.init(myInitializer)
const Logger = stampit.init([myInitializer])
const Logger = stampit.initializers(myInitializer)
const Logger = stampit.initializers([myInitializer])
​
const Logger = stampit().init(myInitializer)
const Logger = stampit().init([myInitializer])
const Logger = stampit().initializers(myInitializer)
const Logger = stampit().initializers([myInitializer])
Last modified 4yr ago