# what is callback

function runDoc(callback) {
// may be task top
callback() // may be callback function call at top

// task middle
callback() // may be in call middle

// task done/end
callback() // may be call in end (final run with final value)
}

runDoc(callback);

# Example

function eachDouble(arr, callback) {
for (let i = 0; i < arr.length; i++) {
callback(arr[i] * 2);
}
return callback;
}

const arr = [10, 1, 2, 3, 4, 5];
const showValue = (value) => console.log(value);
eachDouble(arr, showValue);

here in this function console.log(value) is a callback;
const showValue = (value) => console.log(value);