async-throttle

Throttle async and promise returning functions. Unlike similarly named packages, this behaves much like an async version of lodash.throttle:

• Only one invocation can be running at a time (similarly named packages don't do this)
• Has .cancel() and .flush()

Differences from lodash.throttle

• No leading and trailing options
• getNextArgs option allows you to customize how the arguments for the next invocation are determined

Installing

npm install --save @jcoreio/async-throttle

Usage

const throttle = require('@jcoreio/async-throttle')
import throttle from '@jcoreio/async-throttle'

function throttle<Args: Array<any>, Value>(
  func: (...args: Args) => Promise<Value>,
  wait: ?number,
  options?: {
    getNextArgs?: (current: Args, next: Args) => Args
  }
): (...args: Args) => Promise<Value>;

Creates a throttled function that only invokes func at most once per every wait milliseconds, and also waits for the Promise returned by the previous invocation to finish (it won't invoke func in parallel).

The promise returned by the throttled function will track the promise returned by the next invocation of func.

If wait is falsy, it is treated as 0, which causes func to be invoked on the next tick afte the previous invocation finishes.

By default, func is called with the most recent arguments to the throttled function. You can change this with the getNextArgs option -- for example, if you want to invoke func with the minimum of all arguments since the last invocation:

const throttledFn = throttle(foo, 10, {
  getNextArgs: ([a], [b]) => [Math.min(a, b)],
})
throttledFn(2)
throttledFn(1)
throttledFn(3)
// foo will be called with 1

// time passes...

throttledFn(4)
throttledFn(6)
throttledFn(5)
// foo will be called with 4

throttledFn.invokeIgnoreResult(args)

Calls the throttled function soon, but doesn't return a promise, catches any CanceledError, and doesn't create any new promises if a call is already pending.

To use this, you should handle all errors inside the throttled function:

const throttled = throttle(async (arg) => {
  try {
    await doSomething(arg)
  } catch (err) {
    // handle error
  }
})

Then call invokeIgnoreResult instead of the throttled function:

throttled.invokeIgnoreResult(arg)

The invokeIgnoreResult method is useful because the following code example would leave 1000 pending promises on the heap, even though the catch block is a no-op:

for (let i = 0; i < 1000; i++) {
  throttled(arg).catch(() => {})
}

throttledFn.cancel()

Cancels the pending invocation, if any. All Promises tracking the pending invocation will be rejected with a CancelationError (const {CancelationError} = require('@jcoreio/async-throttle')). However, if an invocation is currently running, all Promises tracking the current invocation will be fulfilled as usual.

Returns a Promise that will resolve once the current invocation (if any) is finished.

throttledFn.flush()

Cancels the wait before the pending invocation, if any. The pending invocation will still wait for the current invocation (if any) to finish, but will begin immediately afterward, even if wait has not elapsed.

Returns a Promise that will resolve once the current invocation (if any) is finished.