Kevin Powell - Community•5mo ago

runtime of loops

Why that variation occurs between for, for...of and forEach? I'm posting here because I was running it in a "nodejs env"

5 Replies

Pi (I'll eat who dont correct me•5mo ago

Code:

const iterations = 1000000;

const myArray = Array.from(Array(iterations).keys());
 let total1 = 0;
 const start1 = performance.now(); // Looping method will be here

   for (let i = 0; i < myArray.length; i++) {
       total1 += myArray[i];
   }
    
 const ends1 = performance.now();


console.log(`for: It took ${ends1 - start1} ms. `);

 let total2 = 0;
 const start2 = performance.now(); // Looping method will be here

   for (const item of myArray) {
       total2 += item;
   }
    
 const ends2 = performance.now();



console.log(`for-of: It took ${ends2 - start2} ms. `);

 let total3 = 0;
 const start3 = performance.now(); // Looping method will be here

   myArray.forEach((data) => total3 += data)
    
 const ends3 = performance.now();



console.log(`forEach: It took ${ends3 - start3} ms. `);

const iterations = 1000000;

const myArray = Array.from(Array(iterations).keys());
 let total1 = 0;
 const start1 = performance.now(); // Looping method will be here

   for (let i = 0; i < myArray.length; i++) {
       total1 += myArray[i];
   }
    
 const ends1 = performance.now();


console.log(`for: It took ${ends1 - start1} ms. `);

 let total2 = 0;
 const start2 = performance.now(); // Looping method will be here

   for (const item of myArray) {
       total2 += item;
   }
    
 const ends2 = performance.now();



console.log(`for-of: It took ${ends2 - start2} ms. `);

 let total3 = 0;
 const start3 = performance.now(); // Looping method will be here

   myArray.forEach((data) => total3 += data)
    
 const ends3 = performance.now();



console.log(`forEach: It took ${ends3 - start3} ms. `);

Jochem•5mo ago

I don't know too much about the internal workings of JS, but the foreach calls a callback function rather than just looping over a block of code. That's got more overhead. I'm assuming for-of has a similar issue

Pi (I'll eat who dont correct me•5mo ago

What is even more weird is that the results also changes depending on the increase or decrease of amount of iterations For 10.000 iterations, forEach was the winner

Jochem•5mo ago

I would mostly like to point out that, while it's fine to wonder about this stuff and want to know the reasons, it's not worth thinking about when you're writing production code no one is going to notice a 15ms delay in processing, and you're just measuring the overhead of the loop anyway, the real slowdown would come from the potentially complicated body of the loop (however you run it) rather than the mechanism of the loop itself what matters much, much more than this level of performance futzing, is readable code. Write what you think is readable, and worry about the performance if you see a hit. You can always profile your code and see where the slowdown happens if it's ever noticeable

Pi (I'll eat who dont correct me•5mo ago

It makes sense

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runtime of loops