javascript - How can I set a default property for an object that's passed into a constructor? -

i'm trying make change in 1 spot affect config object passed instantiations of object. object made available globally follows:

function crayons(){   return {     foo: thirdpartyfoo   } } 

the object initialized in project var myfoo = new crayons().foo({color: "red"});

i'd make {color: "blue"} default, if doesn't pass in color, blue set.

i tried doing

function crayons(){   var foowithdefaults = function(){     = new thirdpartyfoo(arguments); //this invalid     this.color = "blue"; //and overwrite color if set   }    return {     foo: foowithdefaults   } } 

but new keyword throwing me off, don't know how create javascript constructor says this = new 3rdpartyfoo.

what missing?

you can either decorate constructor:

function crayons(){   function foowithdefaults() {     3rdpartyfoo.apply(this, arguments); // you're looking     if (!this.color) // or whatever detect "not set"       this.color = "blue";   }   foowithdefaults.prototype = 3rdpartyfoo.prototype; // make `new` work    return {     foo: foowithdefaults   } } 

or make factory returns instance:

function crayons(){   function foowithdefaults(arg) {     var = new 3rdpartyfoo(arg); // should know how many arguments takes     if (!that.color) // or whatever detect "not set"       that.color = "blue";     return that;   }    return {     foo: foowithdefaults   } } 

here can drop new when calling var myfoo = crayons.foo({color: "red"});

an alternative modifying instance after creation decorate options passed in, in general better solution:

function crayons(){   function foowithdefaults(arg) {     if (!arg.color) // or whatever detect "not set"       arg.color = "blue";     return new 3rdpartyfoo(arg);   }    return {     foo: foowithdefaults   } }