I am a bit confused with the output. Tried in Javascript

var x = 1;
x = x++;
console.log(x); //Its output is 1

I am a bit confused with the output. Tried in Javascript

var x = 1;
x = x++;
console.log(x); //Its output is 1

I was thinking it to be 2. because I am doing the print after the post-increment. Any views on it?

• javascript
• operators
• operator-precedence
• post-increment

• 5 The post in post-increment means it first returns the current value and post that increments the variable. And you’re then assigning the returned value back to the variable… – deceze ♦ Commented Apr 4, 2019 at 11:53
• 3 it's because you're assigning the value of x++ back into x. x++ returns the value of x before the increment, while ++x returns the value afterwards. That's the only difference. – Robin Zigmond Commented Apr 4, 2019 at 11:53
• Does this answer your question? Postfix and prefix increments in JavaScript – Sebastian Simon Commented Mar 9, 2021 at 13:09

2 Answers 2

The order in which x = x++ is executed is as follows:

• Old value of x is calculated (oldValue = 1)
• New value for x is calculated by adding 1 to old value (newValue = 2)
• New value is assigned to x. At this point x bees 2!
• Old value is returned (return value is 1). This concludes the evaluation of x++
• The old value is assigned to x. At this point x bees 1

The above rules are described here. The rules indicate that x is incremented before assignment, not after.

It's correct. The assignment goes first, then the incrementing. Compare:

var x = 1
var y = 1
x = x++
y = ++y

console.log(x, y)