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JavaScript | Passing parameters to a callback function

  • Difficulty Level : Basic
  • Last Updated : 29 Jun, 2020

Callback Function:
Passing a function to another function or passing a function inside another function is known as a Callback Function.
In other words, a callback is an already defined function which is passed as an argument to the other code


function geekOne(z) { alert(z); }
function geekTwo(a, callback) {
prevfn(2, newfn);

Above is an example of a callback variable in JavaScript function.
“geekOne” accepts an argument and generates an alert with z as the argument.
“geekTwo” accepts an argument and a function.
“geekTwo” moves the argument it accepted to the function to passed it to.
“geekOne” is the callback function in this case.


function GFGexample(fact, callback){ 
  var myFact = "GeeksforGeeks Is Awesome, " + fact;
  callback(myFact); // 2
function logFact(fact){
GFGexample("Learning is easy since", logFact);


GeeksforGeeks Is Awesome, Learning is easy since

In this, The “GFGexample” is the main function and accepts 2 arguments, the “callback” is the second one. The logFact function is used as the callback function. When we execute the “GFGexample” function, observe that we are not using parentheses to logFact since it is being passed as an argument. This is because we don’t want to run the callback spontaneously, we only need to pass the function to our main function for later execution.
Make sure that if the callback function is expecting an argument. Then we supply those arguments while executing.
Moreover, you don’t need to use the word “callback” as the argument name, JavaScript only needs to know it’s the correct argument name.

JavaScript callback functions are easy and efficient to use and are of great importance Web applications and code.

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