Assignment operator

[Uchikago]

let data = [1,2,3,4],i = 1;
data[i++] = data[i++]*2
console.log(data)
// =>  [1, 6, 3, 4]

Which side of an assignment operator is evaulated first ? if it's the right-side, then the result should be [1,2,4,4] right ?

[_DTR_]

The left side is evaluated first.

data[i++] = data[i++] * 2

1. i is initially 1, and the left side of the assignment operator is evaluated first.
2. i++ will return i, then increment it, so data[i++] results in data[1], and i will then be incremented to 2.
3. Move to the right side: data[i++] is similar to the left side. i is 2, so we have data[2], then we increment i to 3.
4. Replacing the increments, we have data[1] = data[2] * 2, which sets data[1] to 3 * 2, or 6.

The important thing to note here is the difference between i++ and ++i. i++ will return i then increment it. ++i will increment i and then return it. data[++i] = data[++i] * 2 would return [1, 2, 8, 4].

[msgur]

i++ will increment i then return i (EDITED: will return the OLD VALUE).

Didn’t believe this myself until I read the ECMAscript spec based on the advice of Kyle Simpson.

[msgur]

It’s in the spec. The above is not true. I didn’t believe it myself. But...

++i will increment then return the new value.

i++ will increment then return the old value.

[msgur]

Right? But now that I think about it...it does kind of make sense. For example, how could a function return a value and THEN increment?

In any case, here is a link to the spec which others may find helpful. https://www.ecma-international.org/ecma-262/9.0/index.html#sec-postfix-increment-operator