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javascriptfunctionhigher-order-functions
Venkatesh Muthyla
Venkatesh Muthyla

Reputation: 89

callback explanation in javascript

In The following code i am using setTimeout to call the foo

function foo() {
  console.log('2000 milliseconds have passed since this demo started');
  }
  setTimeout(foo, 2000);

According to my knowledge if we won't use the parenthesis the code will just refer to the function as a pointer how the function is being an executed called here without parenthesis

Upvotes: 0

Views: 62

Answers (2)

Ammar Hussain
Ammar Hussain

Reputation: 304

Take an example 

function f() {
    return 10;
}

var a = f();
var b = f;

Here a contains 10 while b will contain a reference to function f. You can call the function on b using 

var c = b(); // c contains 10

In a similar way settimeout works. If you call the function with parentheses it will get called immediately. While we dont use parentheses so the settimeout function will have a reference and call it after the specified time.

Upvotes: 1

Quentin
Quentin

Reputation: 943214

Take this example for comparison:

function myFunction(callback) {
    callback();
}

function foo() {
  console.log('The foo function has been called');
}

myFunction(foo);

The function you pass as an argument is called by code inside myFunction.

setTimeout works in a similar way: You just aren't looking at its source code because it isn't code you wrote yourself. It comes with the browser.

Upvotes: 1

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