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JavaScript Asynchronous Programming with JavaScript Asynchronous JavaScript with Callbacks Callbacks Review

George Roberts
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George Roberts
Full Stack JavaScript Techdegree Student 8,474 Points

If callbacks only run once the parent completes, why do statements coming after a callback in a parent's body still run?

Hi, when add is called below, the callback executes and then 'still running' is logged to the console. I don't understand how that can be if callbacks only run once the parent has completed. In this case it seems the parent is still running otherwise 'still running' would not be logged after the sum of 2 + 4 is logged? Thanks

function add(a, b, callback) {
  callback(a + b);
  console.log("still running");
}

add(2, 4, function(sum) {
  console.log(sum); 
});
// first logs 6 (callback executes)
// then logs 'still running' (2nd statement in the parent function)

1 Answer

Steven Parker
Steven Parker
228,991 Points

The notion that "callbacks only run once the parent has completed" isn't correct. There are certainly some cases in asynchronous programming where they do happen to run after the parent has completed, but they don't only work that way.

In this particular code the callback is synchronous, so it runs first and then control returns to the parent.