javascript - Behavior of 'return' and 'new' in constructors -

i wonder what's difference between these 2 calls:

var = array(1,2,3); var b = new array(1,2,3); 

i tried mimic behavior way:

function test() {   return {x:42}; }  var = test(); var b = new test(); console.log(a,b); // same 

how possible b same a? if new used, return value should ignored:

function test() {   return 1; }  var = test(); var b = new test(); console.log(a,b); // 1 test{} 

when call function using new, new object created , passed function this. if function doesn't return (or returns primitive), new object passed in result of new expression. but, if function returns object, new object created thrown away , object returned function result of new expression instead. covered in turgid prose in section 11.2.2 of spec. it's useful immutable objects constructor maintains kind of cache , reuses instances if arguments identical (or of course, singletons).

so in example, since test function returns object, calling via new creates , throws away object created new operator, , result object you're returning test instead.

i shouldn't object new "thrown away" if return different object. constructor might object new keeps being thrown away. instance, write constructor this:

function weirdconstructor() {     return {         thenewobject:     }; } 

...which takes object created new , makes property of object returns. call special scenario put mildly. :-)

the fact array(1, 2, 3) , new array(1, 2, 3) both return new array special feature of array constructor, covered section 15.4.1. spec lays out each of standard constructors if don't call them via new. behavior varies. many of them type coercion, couple array act used new though didn't, , date weird thing (gives string version of current date, string(new date())).

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specifically regarding array: wouldn't use either array(1, 2, 3) or new array(1, 2, 3). i'd use [1, 2, 3]. it's not ambiguous (does new array(3) create blank array length = 3 or array 1 entry, 3, in it? new array("3")?), , it's possible (though unlikely) shadow symbol array, can't shadow literal form. don't think point of question. :-)