How lambda function parameter is passed to function body

Question

I am new to javacscript and typescript, I found some code online, I do not understand it, can you explain it.

Thanks

original code https://www.learnrxjs.io/learn-rxjs/operators/error_handling/retrywhen

export const genericRetryStrategy = ({
  maxRetryAttempts = 3,
  scalingDuration = 1000,
  excludedStatusCodes = []
}: {
  maxRetryAttempts?: number,
  scalingDuration?: number,
  excludedStatusCodes?: number[]
} = {}) => (attempts: Observable<any>) => {
  return attempts.pipe(
    mergeMap((error, i) => {
      const retryAttempt = i + 1;
      // if maximum number of retries have been met
      // or response is a status code we don't wish to retry, throw error
      if (
        retryAttempt > maxRetryAttempts ||
        excludedStatusCodes.find(e => e === error.status)
      ) {
        return throwError(error);
      }
      console.log(
        `Attempt ${retryAttempt}: retrying in ${retryAttempt *
          scalingDuration}ms`
      );
      // retry after 1s, 2s, etc...
      return timer(retryAttempt * scalingDuration);
    }),
    finalize(() => console.log('We are done!'))
  );
};

First question, how does this piece of code work

export const genericRetryStrategy = ({
  maxRetryAttempts = 3,
  scalingDuration = 1000,
  excludedStatusCodes = []
}: {
  maxRetryAttempts?: number,
  scalingDuration?: number,
  excludedStatusCodes?: number[]
} = {})

second question, there is **=> (attempts: Observable<any>)** => between code above and function body, how **maxRetryAttempts** is pass to function body.

what part of it don't you understand? Be a little more clearer so we can help better. — seriously, Nov 13 '21 at 23:16
Maybe this could help you. https://stackoverflow.com/questions/34696686/what-is-lexical-this — Honsemiro, Nov 14 '21 at 03:31

score 1 · Answer 1 · answered Nov 14 '21 at 06:50

export const genericRetryStrategy = ({
  maxRetryAttempts = 3,
  scalingDuration = 1000,
  excludedStatusCodes = []
}: {
  maxRetryAttempts?: number,
  scalingDuration?: number,
  excludedStatusCodes?: number[]
} = {})

This declare a function that takes one argument. That part after the : on line 5 types that object as an object with three properties, each of which might be undefined or omitted (thats what the ? means).

In lines 2-4 we see that this object is descructured, so that each property becomes a local variable. In addition, each variable has a default value (the = 3 after the variable name) in case that property has no value on the argument object.

The last line provides a default value for the whole object, meaning you can omit it completely.

Now for this line:

//...
} = {}) => (attempts: Observable<any>) => {

In short, this function returns a function.

A simpler example:

you could write this:

const a = (x: number) => {
  return (y: number) => x * y
}
const b = a(2)
b(3) // 6

const a = (x: number) => (y: number) => x * y
const b = a(2)
b(3) // 6

In plain english, a is a function which, returns a function, which returns a value.

In the case of the code in your question genericRetryStrategy takes an objects that configures how the operation will retry and returns a function. That function that takes an observable to operate on, which reties according how you first called genericRetryStrategy

So to sum up:

export const genericRetryStrategy = ({
  maxRetryAttempts = 3,
  scalingDuration = 1000,
  excludedStatusCodes = []
}: {
  maxRetryAttempts?: number,
  scalingDuration?: number,
  excludedStatusCodes?: number[]
} = {}) => (attempts: Observable<any>) => {
  //..
}

is called like this:

const strategy = genericRetryStrategy({ maxRetryAttempts: 10 })
const doWithRetries = strategy(myObservable)

How lambda function parameter is passed to function body

1 Answers1