Basic callback in Javascript

Question

How can I implement this code to run in correct order (A and B) without change the setTime using only callback?

function a() {
  setTimeout(function () {
    console.log('a');
  },1000);
}

function b() {
  console.log('b');
}

function c() {
  a();
  b();
}

c();

When run the c(), I need to run a() and b() in synchronous order, that's:

wait a sec;
run a() function...
... and then, run b() function

Who do you mean by "without change the setTime using only callback?"? — guest271314
– guest271314, Commented Mar 1, 2017 at 2:41
That's sequential, not "synchronous". setTimeout is always asynchronous. — Bergi
– Bergi, Commented Mar 1, 2017 at 2:43

nnnnnn · Accepted Answer · 2017-03-01 02:42:35Z

3

You mentioned using a callback - simply add an argument to a() that is a callback function that you will call after doing the console.log(a), then when you call a() pass a reference to b:

function a(callback) {
  setTimeout(function () {
    console.log('a');
    if (typeof callback === 'function')
      callback();
  },1000);
}

function b() {
  console.log('b');
}

a(b);   // note: no parentheses after b, i.e., a(b), *not* a(b())
a();    // note: a() still works without any argument

The test within a() to check that callback actually is a reference to a function means you can choose not to pass a callback and a() will still work.

answered Mar 1, 2017 at 2:42

nnnnnn

150k30 gold badges210 silver badges247 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

David · Accepted Answer · 2017-03-01 02:42:41Z

1

The asynchronous operation (what's being done in setTimeout()) would need a callback to invoke when it's finished. Provide that callback to a():

function a(callback) {
  setTimeout(function () {
    console.log('a');
    if (typeof callback === "function") { 
      callback();
    }
  },1000);
}

Then pass a reference to b for that callback:

function c() {
  a(b);
}

answered Mar 1, 2017 at 2:42

David

221k42 gold badges245 silver badges337 bronze badges

1 Comment

Biruel Rick Over a year ago

Great David. Thanks so much!

Collectives™ on Stack Overflow

Basic callback in Javascript

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related