CODEWITHSUNDEEP
javascriptcallback
Biruel Rick
Biruel Rick

Reputation: 786

Basic callback in Javascript

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:

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

Upvotes: 0

Views: 52

Answers (2)

David
David

Reputation: 219127

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);
}

Upvotes: 1

nnnnnn
nnnnnn

Reputation: 150080

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.

Upvotes: 3

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