10 November 2021

A Closure Case

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment) In other words, a closure gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is created, at function creation time.

Closures can be used in the scenario of a try-catch block where you want a method to be called inside both of the scopes, depending on the return value of an execution.

In the following snippet, we need to invoke publish() but with a set of common values except for status , which would be the result of the method invoked within the try-catch block.

We can totally go ahead with invoking publish with the values repeated in both the try-catch scopes like below

try {
  const result: Result = await anAsyncMethod();
  await publish(
    IDENTIFIER, {
      type: request_type,
      requestStatus: REQUEST_STATUS.SUCCESS
    }
  )
  return {
    error: null,
    value: result.data
  }
} catch (err) {
  await publish(
    IDENTIFIER, {
      type: request_type,
      requestStatus: REQUEST_STATUS.FAILURE
    }
  )
  throw err;
}

This results in repeated code - which is fine but we can go one step and refactor this with closures.

try {
  const result: Result = await anAsyncMethod();
  await publishStatus(REQUEST_STATUS.SUCCESS)();
  return {
    error: null,
    value: result.data
  }
} catch (err) {
  await publishStatus(REQUEST_STATUS.FAILURE)();
  throw err;
}

The publishMetric function will be like below

const publishStatus = (status: REQUEST_STATUS) => {
  return async () => publish(
    IDENTIFIER, {
      type: request_type,
      requestStatus: status
    }
  )
}

Closures are quite useful like this and can help us in many scenarios when dealing with asynchronous situations.

Reference

• Closures