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Node.js Promise setTimeout resolves quicker than expected

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6

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.

javascript node.js

share|improve this question

edited Mar 7 at 4:36

Charlie Fish

asked Mar 7 at 1:29

Charlie FishCharlie Fish

5,72563177

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Is there a reason you're testing this? I can see testing to make sure that timeout creates a promise and calls a stubbed out setTimeout with the correct params, but it's not your responsibility to ensure the behavior of a core feature like setTimeout.
  
  – Nicholas Tower
  Mar 7 at 1:33
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @NicholasTower It's still logic I'm writing. It would be a dumb mistake, but if I ever were to put ms / 2 instead of just ms or not actually call resolve, I'd want to try to have my tests catch it. Obviously those are dumb situations, but to me, the more tests the better. Even if it is irrelevant, I think the question itself is still relevant. How do I know setTimeout is really working if I'm getting this inconsistent behavior? So in some regard the test proved at least a bit useful and is causing me to dig deeper. Which IMO is valuable.
  
  – Charlie Fish
  Mar 7 at 1:44
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What OS are you running the test on? Perhaps the underlying syscall is to blame: frenchfries.net/paul/dfly/nanosleep.html
  
  – ııı
  Mar 7 at 1:45
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  FWIW too, you can see it at work here, even with Date objects (although these tend to be rounded to 100 in some very-close-to-100 cases). It also works here in the browser - anyway on my Firefox - using only Date objects. No idea why it behaves like this though.
  
  – Stock Overflaw
  Mar 7 at 3:41
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  It's odd that it's running short. The guarantee is that the timeout will run at or after the set time. The amount of time after is ASAP but could be a long time depending on what else is happening. What version are you using? There has been at least one bug like this in setTimeout but these are pretty old.
  
  – Ouroborus
  Mar 7 at 4:35
  

  
  
  
  

 | 
show 6 more comments

6

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.

javascript node.js

share|improve this question

edited Mar 7 at 4:36

Charlie Fish

asked Mar 7 at 1:29

Charlie FishCharlie Fish

5,72563177

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Is there a reason you're testing this? I can see testing to make sure that timeout creates a promise and calls a stubbed out setTimeout with the correct params, but it's not your responsibility to ensure the behavior of a core feature like setTimeout.
  
  – Nicholas Tower
  Mar 7 at 1:33
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @NicholasTower It's still logic I'm writing. It would be a dumb mistake, but if I ever were to put ms / 2 instead of just ms or not actually call resolve, I'd want to try to have my tests catch it. Obviously those are dumb situations, but to me, the more tests the better. Even if it is irrelevant, I think the question itself is still relevant. How do I know setTimeout is really working if I'm getting this inconsistent behavior? So in some regard the test proved at least a bit useful and is causing me to dig deeper. Which IMO is valuable.
  
  – Charlie Fish
  Mar 7 at 1:44
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What OS are you running the test on? Perhaps the underlying syscall is to blame: frenchfries.net/paul/dfly/nanosleep.html
  
  – ııı
  Mar 7 at 1:45
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  FWIW too, you can see it at work here, even with Date objects (although these tend to be rounded to 100 in some very-close-to-100 cases). It also works here in the browser - anyway on my Firefox - using only Date objects. No idea why it behaves like this though.
  
  – Stock Overflaw
  Mar 7 at 3:41
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  It's odd that it's running short. The guarantee is that the timeout will run at or after the set time. The amount of time after is ASAP but could be a long time depending on what else is happening. What version are you using? There has been at least one bug like this in setTimeout but these are pretty old.
  
  – Ouroborus
  Mar 7 at 4:35
  

  
  
  
  

 | 
show 6 more comments

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.

javascript node.js

share|improve this question

edited Mar 7 at 4:36

Charlie Fish

asked Mar 7 at 1:29

Charlie FishCharlie Fish

5,72563177

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

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);
);

Should wait for given time before resolving:

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

 -99.595337
 +100

share|improve this question

edited Mar 7 at 4:36

Charlie Fish

asked Mar 7 at 1:29

Charlie FishCharlie Fish

5,72563177

share|improve this question

edited Mar 7 at 4:36

Charlie Fish

asked Mar 7 at 1:29

Charlie FishCharlie Fish

5,72563177

asked Mar 7 at 1:29

Charlie FishCharlie Fish

5,72563177

asked Mar 7 at 1:29

Charlie FishCharlie Fish

5,72563177