i++ vs ++i: Does It Really Matter in Loops, in modern JavaScript?
If you're a developer who's ever wondered whether to use i++ or ++i in a for loop, you're not alone.
The short answer? In modern JavaScript, it doesn't change a thing for your loop's performance.
Both c++ and ++c simply add 1 to c, and the loop runs at the exact same speed in today's engines like V8 or SpiderMonkey.
Most folks stick with ++c out of habit or coding style, not because it makes a measurable difference here.
But if you're curious about the details, let's dive into the long answer.
We'll break it down step by step, focusing on why this debate even exists and when (if ever) it actually matters.
1. Semantics: Prefix vs. Postfix
At their core, these operators do the same thing—increment a value by 1—but the timing of when they return the value differs:
++c(prefix): Incrementscfirst, then returns the new value.c++(postfix): Returns the old value first, then incrementsc.
In a for loop like for (let i = 0; i < 10; INC), the increment expression (INC) runs at the end of each iteration. Crucially, the return value of INC is discarded—it's not used anywhere. So, both forms end up leaving your counter in the same state after the increment. No difference in behavior for the loop itself.
2. Historical Micro-Optimization (From C/C++)
This whole debate traces back to C and C++, where performance tweaks mattered more in the old days.
In C++, the postfix form (i++) might need to create a temporary copy of the object to return the "old" value before incrementing. For simple primitive integers, this copy is essentially free. But with heavier objects like custom iterators (think STL containers), it could add a few extra instructions.
That's why old style guides preached: "Always prefer ++i for efficiency." It became a habit, enforced by tools like clang-format and followed religiously—even in languages like JavaScript where the overhead doesn't apply. Engineers love consistency, after all.
3. JavaScript Today
Fast-forward to modern JavaScript: Just-in-time (JIT) compilers in engines like V8 (Chrome), SpiderMonkey (Firefox), and JavaScriptCore (Safari) are smart. When the result of the increment isn't used (as in a for loop), they optimize both forms to identical machine code.
Benchmarks confirm it—no measurable speed difference for primitive counters. Whether you're looping over an array or processing data, i++ and ++i perform the same. The historical C++ baggage? Optimized away.
4. When It Does Matter
Outside of isolated increments in loops, the difference shines through when you use the returned value:
let a = c++;//agets the old value ofc, thencincrements.let b = ++c;//cincrements first, thenbgets the new value.
In these cases, choose based on what you need. But for standalone increments (like in loops), they're interchangeable stylistic choices.
Takeaway
Pick whichever you (or your linter) like best. Many devs default to ++i because:
- It reduces the risk of "use the old value" bugs in non-loop code.
- It's a carryover habit from C++ where it can be marginally faster for complex iterators.
- Major style guides (e.g., Google C++, LLVM, Qt, MISRA) recommend it, so it shows up automatically in JavaScript too.
That said, for something like for (; (n & 1) === 0; c++) versus for (; (n & 1) === 0; ++c), the generated bytecode—and runtime performance—are identical.
In the end, focus on writing clear, maintainable code.
Micro-optimizations like this are fun to geek out on, but they're rarely the bottleneck in real-world apps.