Callback after all async forEach callbacks complete

Array.forEachCan't provide this effect (if you can), but there are several ways to achieve what you want:

use a simple counter

function callback () { console.log('all done'); }

var itemsProcessed = 0;

[1, 2, 3].forEach((item, index, array) => {
  asyncFunction(item, () => {
    itemsProcessed++;
    if(itemsProcessed === array.length) {
      callback();
    }
  });
});

(Thanks to @vanuan et al), this approach ensures that all items are processed before the "done" callback is called. You need to use the counter updated in the callback. Relying on the value of the index parameter does not provide the same guarantee, because the return order of asynchronous operations is not guaranteed.

Use ES6 promises

(the promise library is available for older browsers):

1. Process all requests guaranteed to be executed synchronously (e.g. 1 to 2 then 3)

   function asyncFunction (item, cb) {
     setTimeout(() => {
       console.log('done with', item);
       cb();
     }, 100);
   }
   
   let requests = [1, 2, 3].reduce((promiseChain, item) => {
       return promiseChain.then(() => new Promise((resolve) => {
         asyncFunction(item, resolve);
       }));
   }, Promise.resolve());
   
   requests.then(() => console.log('done'))
   

2. Process all asynchronous requests without "synchronous" execution (2 may finish faster than 1)

   let requests = [1,2,3].map((item) => {
       return new Promise((resolve) => {
         asyncFunction(item, resolve);
       });
   })
   
   Promise.all(requests).then(() => console.log('done'));
   

Use async library

There are other async libraries ( async being the most popular) that provide mechanisms to express what you want.

The body of the question was edited to remove the previously synced sample code, so I've updated the answer to clarify. The original example uses synchronous-like code to model asynchronous behavior, so the following applies:

array.forEachis synchronous, too res.write, so you just put the callback after calling the foreach:

  posts.foreach(function(v, i) {
    res.write(v + ". index " + i);
  });

  res.end();