Why does two small for-loops run faster than a big one ?

#explainlikeimfive #discuss #javascript

Explain Like I am five Why does two small for loops run faster than one big for loop?

s = Date.now()
for(let i=0;i<1e10;i++) { i + 100 }
for(let i=0;i<1e10;i++) { i + 100 }
console.log(Date.now() - s)
// => 37s


s = Date.now()
for(let i=0;i<2e10;i++) { i + 100 }
console.log(Date.now() - s)
// => 38s

Especially for language like JS which is not multi-threaded

Top comments (4)

da-ti • Aug 27 '19 • Edited

This has nothing to do with JIT compiler, loops, multi-threading or BigInt. If this was a some quiz question it would test:
-Understating that every number in js implicitly is floating point number
-Knowing how floating point addition works and that (unlike addition of integers) it takes multiple hw level operations

Addition_and_subtraction

To make this test fair for both one loop variant and two loops variant:


s = Date.now()
for(let i=0;i<1e10;i++) { i + 100 }
for(let i=1e10;i<2e10;i++) { i + 100 }
console.log(Date.now() - s)


s = Date.now()
for(let i=0;i<2e10;i++) { i + 100 }
console.log(Date.now() - s)

Now both variants work with exact same numbers and perform exact same floating point operations (time diff is consistently <20ms on my machine now, where it was 500-1500ms before)

TLDR: floating point