Js magic need explanation

Question

I'm trying to solve this magic in js:

var a = 1;
console.log(a++ + ++a);

It returns 4 and I can understand that it's 1+3 but what is the sequence of this?

If you understand what post-increment and pre-increment operators do, it becomes pretty obvious. You just need to read the docs and the knowledge that code is executed from left to right. — Sebastian Simon, Sep 26 '16 at 19:30
I do yet it doesn't seem obvious. Can you just describe it like 1)... 2)... 3)... in your answer? — Vadym, Sep 26 '16 at 19:31
If you see code like this in real life, then it was written by a horrible person. There are good reasons to use `++a` or `a++` depending on the scenario, but having 5 plus signs in a row for any reason is a bad idea for readability, and is just asking for a typo. — Joe Enos, Sep 26 '16 at 19:36

score 5 · Accepted Answer · answered Sep 26 '16 at 19:35

a++ means return the value before incrementing

++a means returns the value after incrementing

So to break down your example:

var a = 1;
console.log(a++ + ++a);

a is set to 1
a++ returns 1, and then a is incremented to 2 (but we don't do anything with it at this point)
++a increments a and returns 3
Then altogether we have console.log(1 + 3) with the 1 coming from step 1 and the 3 coming from step 3.

score 3 · Answer 2 · answered Sep 26 '16 at 19:36

what is the sequence of this

In pseudo spec language:

Evaluate addition (a++ + ++a)
1. Let lval be the result of evaluating the left operand (a++)
  1. Get the value of a and assign it to oldValue. (1)
  2. Let newValue be oldValue + 1.
  3. Assign newValue to a. (a == 2)
  4. Return oldValue. (1)
2. Let rval be the result of evaluating the right operand (++a)
  1. Get the value of a and assign it to oldValue. (2)
  2. Let newValue be oldValue + 1.
  3. Assign newValue to a. (a == 3)
  4. Return newValue. (3)
3. Return lval + rval. (1 + 3)
Result: 4

score 1 · Answer 3 · edited May 23 '17 at 10:27

To go into a bit more detail on what Xufox was saying in the comments section:

++a first increments the value of a and then returns an lvalue referring to a, so if the value of a is used then it will be the incremented value.

a++ first returns an rvalue whose value is a, that is, the old value, and then increments a at an unspecified time before the next full-expression (i.e., "before the semicolon").

Postfix increment has higher precedence than prefix increment.

This helped me a lot back in the day and it should help you too..

http://en.cppreference.com/w/cpp/language/operator_incdec

Js magic need explanation

3 Answers3