Why is
++[[]][0] == 1
But does
++[]
throw an error
Aren't they the same? I would believe the first example executes an index-read on the array so you get the array in the array. And then the increment is executed. If so than why can't I do the second example?
++ is an assignment operator. It requires a valid left-hand-side operand (even though it can be on the right of ++).
[] is only a value, not something you can assign to.
[[]][0] evaluates to [] but it is a valid left-hand-side, because it points to an element in an existing array. So that works.
To give a hopefully less confusing example:
var a = 1
1++ // throws an error
a++ // works fine
It doesn't matter what value is in a. Worst case, ++ will return NaN, never an error, as long as it can assign the result.
The only JavaScript quirkiness in your example is that +[] + 1 evaluates to 1 because the empty array is coerced to an empty String, then explicitly to zero (+"" is 0), which is then added to 1.
The ++ operator always coerces to number, unlike + which would be satisfied with "" (so [] + 1 turns into "" + "1"). Thus, when decomposing a ++, don't forget to force the operands to number (not that it ultimately matters in your example).
++[[]][+[]] [[]][+[]] = [[]][+[]] + 1; [[]][0] = [[]][0] + 1; [[]][0] = [] + 1; [[]][0] = "" + "1"; // +-operator prefers string concatination, empty array = "" ["1"] // the first value of the array is replaced by "" + "1"
+[]) :p You got the evaluation order mostly right, except that the top priority is actually +[] which evals to zero before anything else.++ coerces to number (unlike += 1 which is truly equivalent to = X + 1) so it doesn't happen exactly the same as in my post. In your "puzzle" here, the first part evals to the number 1 (by coercing the empty array to "" and then to 0, and adding 1) rather than the string "1". It is then coerced to a string because the second part ([0]) is coerced to "0".++.