# 30秒学会 JavaScript 片段 – hz

Returns the number of times a function executed per second.
`hz` is the unit for `hertz`, the unit of frequency defined as one cycle per second.

Use `performance.now()` to get the difference in milliseconds before and after the iteration loop to calculate the time elapsed executing the function `iterations` times.
Return the number of cycles per second by converting milliseconds to seconds and dividing it by the time elapsed.
Omit the second argument, `iterations`, to use the default of 100 iterations.

#### 代码片段

``````const hz = (fn, iterations = 100) => {
const before = performance.now();
for (let i = 0; i < iterations; i++) fn();
return (1000 * iterations) / (performance.now() - before);
};``````

#### 使用样例

``````// 10,000 element array
const numbers = Array(10000)
.fill()
.map((_, i) => i);

// Test functions with the same goal: sum up the elements in the array
const sumReduce = () => numbers.reduce((acc, n) => acc + n, 0);
const sumForLoop = () => {
let sum = 0;
for (let i = 0; i < numbers.length; i++) sum += numbers[i];
return sum;
};

// `sumForLoop` is nearly 10 times faster
Math.round(hz(sumReduce)); // 572
Math.round(hz(sumForLoop)); // 4784``````