Behaviour of commas within parentheses in Javascript

Question

I have a twofold question which involves something I would consider to be incorrect Javascript code.

How is the following statement interpreted in Javascript, and why?

(1,2,3,4)

Why is there a difference between these two invocations:

var a = (1,2,3,4);
a();

which leads to a being equal to 4 and Uncaught TypeError: a is not a function being thrown, and

(1,2,3,4)();

which leads to Uncaught TypeError: (((1 , 2) , 3) , 4) is not a function?

Answer 1

How is the following statement interpreted in Javascript, and why?

(1,2,3,4)

That's a comma operator expression (actually, a chain of them) wrapped in grouping parentheses. The comma operator is quite unusual: It evalutes both of its operands, then takes the value of the second one as its value, throwing away the value of the first one. You have a chain of them there, so the value of 1 is evaluated, then 2, then 3, then 4, and the result of the comma operator chain is the value 4; the result of the grouped parentheses expression is therefore 4.

Why is there a difference between these two invocations:

var a = (1,2,3,4);
a();

Because of the syntax of the language. In the first case, it's clearly not a function call, as there's no value prior to the first ( to call. The parsing rules for a complex language like JavaScript are just that: Complex. The parser is context-sensitive, and knows how to differentiate between grouping parentheses and function-call parentheses.

which leads to a being equal to 4 and Uncaught TypeError: a is not a function being thrown, and

(1,2,3,4)();

which leads to Uncaught TypeError: (((1 , 2) , 3) , 4) is not a function?

In both cases, the error message is quoting the expression that yielded the result it then tried to call as a function.

Answer 2

There is an operator in JS (among other languages) called the comma operator. It simply takes two operands, and returns the rightmost one.

a = 1, 2; // a now equals 2

It is, however, not the same comma as the function parameter separator. It is an operator.

Answer 3

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

So, the statement (1,2,3,4); returns 4, so var a = (1,2,3,4); means that a is equal to 4 which, is not a function, thus the error.

Likewise, (1,2,3,4) is just a grouping of comma operators and not a function, thus the second error.