A semaphore implementation using promises. Forked from vercel/async-sema.

• Usage
• API
• Examples
• Contributors
• Changelog

• Universal - Works in all modern browsers, Node.js, and Deno and supports CLI usage.
• Zero Dependencies - Absolutely no dependencies, keeping the package tiny (24kb).
• Tested - Greater than 98% test coverage.
• Typed - Out of the box TypeScript declarations.

<script type="module">
  import { Sema } from 'https://esm.sh/sema4';
</script>

import { Sema } from 'https://esm.sh/sema4?dts';

import { Sema } from 'sema4';

Drains the semaphore and returns all the initialized tokens in an array. Draining is an ideal way to ensure there are no pending async tasks, for example before a process will terminate.

Returns the number of callers waiting on the semaphore, i.e. the number of pending promises.

Attempt to acquire a token from the semaphore, if one is available immediately. Otherwise, return undefined.

Acquire a token from the semaphore, thus decrement the number of available execution slots. If initFn is not used then the return value of the function can be discarded.

Release the semaphore, thus increment the number of free execution slots. If initFn is used then the token returned by acquire() should be given as an argument when calling this function.

Creates a rate limit instance.

Acquires a semaphore and connects a timeout for its release. If the rate limit is reached, the execution process is halted until an available semaphore is released.

Releases all acquired semaphores immediately and resets the timeouts connected to them.

import { Sema } from 'sema4';

function foo() {
  const s = new Sema(
    4, // Allow 4 concurrent async calls
    {
      capacity: 100, // Preallocated space for 100 tokens
    },
  );

  async function fetchData(x) {
    await s.acquire();

    try {
      console.log(s.waiting()   ' calls to fetch are waiting');
      // Perform some async tasks here...
    } finally {
      s.release();
    }
  }

  return Promise.all(array.map(fetchData));
}

import { RateLimit } from 'sema4';

async function bar() {
  const rl = new RateLimit(5); // Limit to 5 tasks per default time interval

  for (let i = 0; i < n; i  ) {
    await rl.apply();
    // Perform some async tasks here...
  }
}

In addition to the contributors of the parent repository vercel/async-sema, these lovely people have helped keep this library going.