javascript - 在 node.js 中，为什么第二次调用函数比第一次花费的时间少得多？

Question

const {performance} = require('perf_hooks');

const nemo = ['nemo'];

function findNemoArray(array) {
  let t0 = performance.now();
  for (let i = 0; i < array.length; ++i) {
    if (array[i] === 'nemo') {
      console.log("Found nemo")
    }
  }
  let t1 = performance.now();
  console.log("Call to find nemo took", t1-t0, "milliseconds");
}

findNemoArray(nemo);
findNemoArray(nemo);

输出:

$ node findNemo.js
Found nemo
Call to find nemo took 6.692564999684691 milliseconds
Found nemo
Call to find nemo took 0.09663600008934736 milliseconds

我不明白为什么第二个电话比第一个电话花费的时间少得多？

Answer 1

这是一个v8引擎优化。
您可以深入阅读 the v8 profiling和 Ignition and Turboban管道(当然也被 Nodejs 使用)

node --prof yourScript.js
# it will produce a log file that you can read by
node --prof-process isolate-00000221B52EB170-26268-v8.log > readable.txt