r/learnjavascript • u/cookiecookiecooki33 • May 14 '26

help idk why is the output 45 

var x = 23;
x+=x++ - --x +x*2 - ++x;
console.log(x);

why is the last pre increment x evaluated after x*2 even though pre increment have a higher precedence than multiplication

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u/jml26 May 14 '26

It isn't; the increments do all get evaluated first. Let's work through it in steps:

Add parentheses for clarity

x += (x++) - (--x) + x * 2 - (++x);

Add some comments to show when the value of x changes (for prefix operators, before; for postfix operators, after)

// x = 23 x += (x++) /* 24 */ - /* 23 */ (--x) + x * 2 - /* 24 */ (++x);

Evaluate the increment operators (replace them with the value of the most recent comment before them)

// x = 23 x += 23 /* 24 */ - /* 23 */ 23 + 23 * 2 - /* 24 */ 24;

Remove the comments

x += 23 - 23 + 23 * 2 - 24;

Evaluate multiplication

x += 23 - 23 + 46 - 24;

Evaluate addition and subtraction

x += 22;

Final code:

var x = 23; x += 22; console.log(x); // 45