Callbacks

Callback functions are functions that are passed as arguments in other functions. A callback function can then be invoked during the execution of that higher order function (that it is an argument of).

In JavaScript, functions can be passed as arguments because functions are objects.

Suppose a function is created which makes a calculation and doubles the results of that calculation:

function doubleResult(num1, num2, calc) {
  return calc(num1, num2) * 2;
}

function add(num1, num2) {
  return num1 + num2;
}

function multiply(num1, num2) {
  return num1 * num2;
}

console.log(doubleResult(4, 2, add));
console.log(doubleResult(4, 2, multiply));

The output will look like this:

12
16

In the code above, doubleResult accepts three arguments num1, num2, and calc. num1 and num2 are the numbers which the user wants to be used in the calculation. calc is a function which the user passes as the third argument into the doubleResult function and contains the calculation code.

add and multiply functions are two basic functions which add or multiply the two numbers passed into them as arguments.

In the log statements, the doubleResult() function is invoked with numbers as first and second arguments. The function add or multiply is passed as the third argument.

In the above example doubleResult is called a higher order function while add and multiply are called callback functions.

Codebyte Example

Here, createNewArray function returns a new array based on array and callback function passed to it as arguments.

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