JavaScript CallBack Function ..ish问题

Question

So I was doing some practice about callback function and I wanted to try it out in my own was and use it with a setTimeout Method and to my surprise, it didn't work as expected.. Pls what am i doing wrong here.

 function first(number, callback) { setTimeout(function() { console.log(number); }, 5); callback(); } function second() { console.log(2); } first(1, second); 

Answer 1

You're executing setTimeout and callback at the same time. Because JavaScript is single-threaded, it doesn't wait for setTimeout to complete before executing the next statement.

That's why, 2 prints immediately, and then 1 prints after a delay of 5 milliseconds.

If you want to print 1 first, then you need to call the callback function in the callback of setTimeout . This ensures that the console.log(number) executes prior to calling the callback (which prints 2 ).

 function first(number, callback) { setTimeout(function() { console.log(number); callback(); }, 5); } function second() { console.log(2); } first(1, second); 

Answer 2

As 31piy already said, setTimeout() and callback() are being executed at the same time. Your code first schedules a function execution at 5ms from now, and then immediately runs another function that logs 2 to the console.

You can achieve the expected result in a couple of similar ways, I'll show one:

 function first(number,callback) { setTimeout(function() { callback(); }, 5); console.log(number); } function second() { console.log(2); } first(1, second); //Here, you immediately print the number 1 and schedule the number 2.