What is callback part of high order function?

JavaScript
#1

I am new to JS and had been learning with HOF/master it under the hood and I have been little confused, it says it calls a callback. but which part of code is callback? e.g.

const multiple = [1,2,3,4,5];
const result = multiple.map((num)=>{
     return num * 2
})

console.log(result)

from my understanding is callback if a function u call inside of a HOF e.g.

function multiplyBy2(array, callback){
  let newArr = []
  for(let i = 0; i < array.length; i++){
     newArr.push(callback(array[i]))
  }
  return newArr
}
function multiply(num){
  return num * 2
}
console.log(multiplyBy2(arr, multiply))

so my question is, what is callback part in higher order function. here the working example of above

#2

this is the callback for that map method - the callback is the function passed as argument of an other function

the function that receive the argument is the higher order function, like map

1 Like
#3

A callback is a function you pass to another function. It’s a term from the days of C: nowadays people often just call them “function parameters”, or even just plain “function”.

1 Like
#4

(num)=>{
return num * 2
}
but that isn’t a function num is a variable where as * 2 is part of map, isn’t it?

#5

that’s a function, num is the function parameter, num*2 is the value returned

the map method uses that function, takes each element of the array, feed it to the callback function, and put the result in the new array

#6

but num is just a instance of each array value we are passing, isn’t it? like a placeholder until the function is called with a arguement in its placeholder.

#7

all function parameters are “placeholders” as you say

consider

function hello(name) {
console.log("Hello " + name)
}

hello("Emily") // it prints "Hello Emily"
#8

so in your example name is a paramater/placeholder but why we call it callback/function paramater in higher order function because from my understanding it is just a placeholder for each array.

I know calling a function inside as a arguement of another function will be called callback? but here i see name in above example and num the same thing?

#9

Maybe it helps to rewrite this with more traditional syntax:

const multiple = [1,2,3,4,5];
const result = multiple.map(function(num){
     return num * 2
})

So this is:

const result = multiple.map(callbackFunc)
1 Like
#10

so many good answers. Thank you all for making me understand. Sorry for getting long to understand.

#11

No worries, we’ve all been there. When learning something new in coding, like HOF, you’ll have to write them at least 50 times in different ways before it really clicks how they work.