what makes the same number of loops have different performance?

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var t1 = new Date().getTime()
for (let i = 0; i < 100; i++) {
  for (let j = 0; j < 1000; j++) {
    for (let k = 0; k < 10000; k++) {
    }
  }
}
var t2 = new Date().getTime()
console.log('first time', t2 - t1)

for (let i = 0; i < 10000; i++) {
  for (let j = 0; j < 1000; j++) {
    for (let k = 0; k < 100; k++) {

    }
  }
}
var t3 = new Date().getTime()
console.log('second time', t3 - t2)

As you can see, it looks like the above two for-loops will have the same execution time. But in fact the second loop will take more time to execute than the first one does. What makes the difference under the hood?

nested-for-loop for-loop javascript performance
1 Answers

In the first loop, you are executing:

  • let j=0; //100 times
  • let k=0; //100*1000=100000 times

In the second loop, you are executing:

  • let j=0; //10000 times
  • let k=0; //10000*1000=10000000 times

The second loop has 9909900 more variable initializations and therefore is expected to run longer.

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