Why does setTimeout with new function ignore wait interval?

When trying to do this:

setTimeout(function(){alert("Boo");}, 500);

I accidentally wrote this:

setTimeout(new function(){alert("Boo");}, 500);

The former version waits 500 millis, then alerts. The latter alerts immediately.

Why does adding new in front开发者_StackOverflow中文版 of the function cause this behavior?

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Using new creates a new object using the anonymous function as its constructor, so your function runs and alerts immediately.

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The latter one instantiates an Object and calls its constructor immediately.

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new function(){alert("Boo");} 

is an equivalent of

var temp1 = function(){alert("Boo");};
var temp2 = new temp1();

second line as you see invokes your function using temp1 as a constructor.