A sample project which demonstrates a few ways of using TypeScript features to avoid common sources of developer error.
This code simulates a system which makes a request on behalf of a user that may fail for a number of different reasons.
In this system, engineers want to monitor the causes of different types of failure in a business-friendly way so that we can get a quick visual understanding of the throughput of our system, and whether something might have changed that affects this - either in our system, or in something that we depend on.
We also want the system's behaviour to be transparent, so that our friendly product owners / managers / etc understand what's going on and feel empowered to make good commercial decisions based on it.
v1.ts demonstrates the application code with no monitoring enabled.
A naive approach, such as v2, adds monitoring but could be prone to typos across multiple invocations of the method. For example, the name of the metric was misspelled somewhere, or a different word was used in the "outcome" field, it would cause continuity issues and make it more difficult to use the data.
As I discovered, unit tests aren't a huge help for this kind of problem, because they suffer the same propensity to typos as regular code!
TypeScript features including union and lookup types can help by locking down the set of legal inputs to the code to only correctly-spelled values, so that you only have to type them out once without assistance.
v3 and v4 show two different possible ways to implement this.
Start the server with yarn start.
Try sending a POST request to any of the /vX endpoints with a body like the following:
{
"search": {
"pickUp": "2021-01-01T09:00:00+00:00",
"dropOff": "2021-01-03T11:00:00+00:00",
"location": "London"
}
}All endpoints except v1 log out a message when a metric is registered. The logs are structured, you can filter out the server logs and get just the metric messages by piping the output into jq:
yarn --silent start | jq 'select(.event == "awesome-corp.brilliant-dept.request")'