Javascript "tuple" notation: what is its point?

Question

At wtfjs, I found that the following is legal javascript.

",,," == Array((null,'cool',false,NaN,4)); // true 

The argument (null,'cool',false,NaN,4) looks like a tuple to me, but javascript does not have tuples!

Some quick tests in my javascript console yields the following.

var t = (null,'cool',false,NaN,4); // t = 4 (null,'cool',false,NaN,4) === 4; // true (alert('hello'), 42); // shows the alert and returns 42 

It appears to behave exactly like a semicolon ; separated list of statements, simply returning the value of the last statement.

Is there a reference somewhere that describes this syntax and its semantics? Why does it exist, i.e. when should it be used?

Answer 1

You are seeing the effect of the comma operator.

The comma operator evaluates both of its operands (from left to right) and returns the value of the second operand.

The resultant value when a,b,c,...,n is evaluated will always be the value of the rightmost expression, however all expressions in the chain are still evaluated (from left to right).

Answer 2

As already explained this behaviour is caused by , operator. Due to this the expression (null,'cool',false,NaN,4) will always evaluate to 4. So we have

",,," == Array(4) 

Array(4) - creates new array with allocated 4 elements. At the time of comparison with the string this array is converted to string like it would be with Array(4).toString(). For arrays toString acts like join(',') method called on this array. So for the empty array of 4 elements join will produce the string ",,,".