javascript Postfix and Prefix Increment in expression - Stack Overflow

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I'm having trouble understanding how work Postfix and Prefix Increment in expression like this:

var x = 1;
x = ++x + x++ * x

Why browser console return 8 ?

I'm having trouble understanding how work Postfix and Prefix Increment in expression like this:

var x = 1;
x = ++x + x++ * x

Why browser console return 8 ?

• javascript
• increment

Share Improve this question Follow asked Mar 13, 2017 at 21:20 Виталий ТВиталий Т 2911 silver badge22 bronze badges 3

• Because 2 + 2 * 3 is 8 – Andrew Li Commented Mar 13, 2017 at 21:22
• 3 Possible duplicate of Why avoid increment ("++") and decrement ("--") operators in JavaScript? – Dave Commented Mar 13, 2017 at 21:23
• 1 You should read up on "operator precedence". – Heretic Monkey Commented Mar 13, 2017 at 21:26

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6 Answers 6

Sorted by: Reset to default 6

It is evaluated left to right:

++x           : x is now 2
++x +         : 2 + 
++x + x       : 2 + 2
++x + x++     : 2 + 2 and x is now 3
++x + x++ *   : 2 + 2 *
++x + x++ * x : 2 + 2 * 3

var x = 1; x = ++x + x++ * x

As per

++x => 2 (After pre-fix's updated value)
x++ => 2 (Post-fix will update in second sequence not now)
x   => 3 (After post-fix's updated value)

so

x = 2 + 2 * 3 

As per multiplication in priority it would

x = 2 + 6

hence

x = 8 

You can learn more at this website: https://developer.mozilla/en-US/docs/Web/JavaScript/Reference/Operators/Increment Here are the reasons:

1. x = 1; X is 1.
2. x = ++x; now x is incremented by 1. (X is 2)
3. x = ++x + x++ * x

Since multiplication is always calculated first, we will multiply x++ by x. Remember, x has value of 2.

This is what looks like in math expression:

x = 1; 
x = 1+1+2+1*3

We will multiply x++ by x. Remember, x is 2. So it will look like this:

x = 1+1 +(2+1{2})

Now we will calculate the numbers inside the pearenthesis.

2+1*2 = 6

After that, we can add 2 to 6 to get 8.

2+6 = 8

This is why.

x = ++x + (x++ * x)

See it as a math operation, form left to right.

++x = x is now 2
x++ = x is now 3 but the showed value remains 2
x = is 3

x = 2 + (2 * 3)
x = 8

According to operator precedence sequence will be executed and returns values by following steps:

++x => 2 // returns current value after increment
x++ => 2 // returns old value, here old value is 2 because already increment before
x++ * x => 2 * 3 => 6  // x returns stored current value after 2 times increment
++x + 6 => 2 + 6 => 8 // Output is 8

NOTE ++x and x++ both increases variable by 1 and store value to the variable(x). But here is simple difference ++x (prefix increment operator) returns current value and x++ (postfix increment operator) returns old value.

Prefix (++variable) When the ++ operator is used before the variable, it increments the variable’s value before the value is used in the expression

let a = 5;
let b = ++a; // a is incremented to 6, then b is assigned the value of 6
console.log(a); // 6
console.log(b); // 6

Postfix (variable++) When the ++ operator is used after the variable, it increments the variable’s value after the value is used in the expression.

let a = 5;
let b = a++; // b is assigned the value of 5, then a is incremented to 6
console.log(a); // 6
console.log(b); // 5

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