micro-promise.js

function Def( arg ) {
  if( arg instanceof Def ) 
    this.parent = arg;
  else if( typeof arg !== "undefined" )
    this.v = arg;
}

Def.prototype = {

  chain: function(v){
    this.v = v;
  },

  resolve: function( v ) {
    if( v instanceof Def )
      v.then( this.resolve.bind( this ) );
    else if( this.cb )
      this.chain( this.cb( v ) );
    else
      this.chain( v );
  },

  then: function( cb ) {
    if( 'v' in this )
      return new Def( cb( this.v ) );
    if( this.parent )
      this.parent.chain = this.resolve.bind( this );

    this.cb = cb;

    return new Def( this );
  }
}

console.log( "Test 1 - starting" );
var a = new Def();
a.then( function(v){ console.log( "Callback:", v ); } )
console.log( "Test 1 - resolving" );
a.resolve( "Value" );

console.log( "Test 2 - starting" );
var a = new Def();
console.log( "Test 2 - resolving" );
a.resolve( "Value" );
a.then( function(v){ console.log( "Callback:", v ); } )

console.log( "Test 3 - starting" );
var a = new Def();
a.then( function(v){ console.log( "Callback 1:", v ); return v; } )
 .then( function(v){ console.log( "Callback 2:", v ); } );
console.log( "Test 3 - resolving" );
a.resolve( "Value" );

console.log( "Test 4 - starting" );
var a = new Def();
console.log( "Test 4 - resolving" );
a.resolve( "Value" );
a.then( function(v){ console.log( "Callback 1:", v ); return v; } )
 .then( function(v){ console.log( "Callback 2:", v ); } );

console.log( "Test 5 - starting" );
var a = new Def();
var b = new Def();
a.then( function(v){ console.log( "Callback:", v ); } )
console.log( "Test 5 - resolving a" );
a.resolve( b );
console.log( "Test 5 - resolving b" );
b.resolve( "Value" );

console.log( "Test 6 - starting" );
var a = new Def();
var b = new Def();
a.then( function(v){ console.log( "Callback 1:", v ); return b; } )
 .then( function(v){ console.log( "Callback 2:", v ); } );
console.log( "Test 6 - resolving a" );
a.resolve( "Value 1" );
console.log( "Test 6 - resolving b" );
b.resolve( "Value 2" );

console.log( "Test 7 - starting" );
var a = new Def();
var b = new Def();
a.then( function(v){ console.log( "Callback 1:", v ); return b; } )
 .then( function(v){ console.log( "Callback 2:", v ); } );
console.log( "Test 7 - resolving b" );
b.resolve( "Value 2" );
console.log( "Test 7 - resolving a" );
a.resolve( "Value 1" );