无法理解javascript中的回调。 请检查

Question

I have written my code to test the callback . but i am getting answer 5 directly for the two calls. I need 2 and 5 respectively. How to achieve it.

var count = 0;

function increment(number, callback) {
    count = count + number;
    setTimeout(callback, 10000);

}

var done = function() {
    console.log(`count is :${count}`);
};
increment(2, done);
increment(3, done);

Answer 1

You're adding the number to count immediately , then scheduling a callback to show the result. That means as soon as the two calls to increment run, count is 5 . Then 20 and 50 seconds later, that is shown. You can see that if we do console.log( count after calls :${count} ); after the two calls (I've lowered the timeouts to 2 and 5 seconds):

 var count = 0; function increment(number, callback) { count = count + number; setTimeout(callback, 1000); } var done = function() { console.log(`count is :${count}`); }; increment(2, done); increment(3, done); console.log(`count after calls :${count}`); 

Instead, you'll want to add number to count in the timer callback (I've lowered the timeouts to 2 and 5 seconds):

 var count = 0; function increment(number, callback) { setTimeout(function() { // Our timer callback fired, update count count += number; // Call the main callback callback(); }, 1000); } var done = function() { console.log(`count is :${count}`); }; increment(2, done); increment(3, done);