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var A = function(x,y){var a=arguments,b=a[0],c=a.callee,d=this;if(!(d instanceof c))return new c(a);if(c==b.callee)return c.apply(d,b);d.x=x;d.y=y;};

 

var a = A(1,2);

console.log(a.x, a.y);        // 1 2

console.log(a instanceof A);  // true

 

var b = new A(3,4);

console.log(b.x,b.y);         // 3 4

console.log(b instanceof A);  // true