setTimeout Calling Callback Too Early

• Version: v8.15.1
• Platform: Darwin Charlies-MacBook-Pro.local 18.2.0 Darwin Kernel Version 18.2.0: Thu Dec 20 20:46:53 PST 2018; root:xnu-4903.241.1~1/RELEASE_X86_64 x86_64

I have the following code in Node.js.

const timeout = (ms) => new Promise((resolve) => setTimeout(resolve, ms));

I'm trying to test this code with the following test:

it("Should wait for given time before resolving", async () => {
    const MS = 100;
    const start = process.hrtime();

    await timeout(MS);

    const diff = process.hrtime(start);
    expect(((diff[0] * NS_PER_SEC) + diff[1]) / 1000000).to.at.least(MS);
});

The problem is sometimes (rarely), this test fails:

Should wait for given time before resolving:

  AssertionError: expected 99.595337 to be at least 100
  + expected - actual

  -99.595337
  +100

Obviously this is some type of timing issue with Node.js or something. If anything I expect await timeout(MS); to take slightly longer than MS. In no case do I expect it to take less time.

What is it about the internals of JavaScript/Node.js that causes this to happen?

This occurred on macOS 10.14.3 running Node.js version 8.15.1.

Start with the timeout helper using setTimeout and the timing assertion using process.hrtime. Compare the observed elapsed time with the requested delay and investigate the Node.js timer behavior described in the issue; the work is complete when the cause is explained or a reproducible correction and validation are identified.