09_JS_Higher-Functions

Software-Engineering

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# What are Higher Order Functions ?

take other function as a parameter or return a function as value. the function passed as parameter is called callback.

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# Callback

A callback can be passed as parameter to other functions

// a callback function, the function could be any name
const callback = (n) => {
  return n ** 2;
};
 
// function takes other function as a callback
function cube(callback, n) {
  return callback(n) * n;
}
console.log(cube(callback, 3));

Output: 27

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# Return Function

Higher order functions return a function as a value

// Higher order function returning an other function
const higherOrder = (n) => {
  const doSomething = (m) => {
    const doWhatEver = (t) => {
      return 2 * n + 3 * m + t;
    }
    return doWhatEver
  }
  return doSomething
}
console.log(higherOrder(1)(2)(3))

Output: 11

For example the forEach method uses callback:

const numbers = [1, 2, 3, 4, 5, 6];
const sumArray = (arr) => {
  let sum = 0;
  const callback = function (element) {
    sum += element;
  };
  arr.forEach(callback);
  return sum;
};
console.log(sumArray(numbers));

Output: 21

A simpler option for the example above would be

const numbers = [1, 2, 3, 4]
​
const sumArray = arr => {
  let sum = 0
  arr.forEach(function(element) {
    sum += element
  })
  return sum
}
console.log(sumArray(numbers))

Output: 21

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# Setting Time

In JS we can execute some activity for a specific time. We can wait for something to execute.

# setIntervall

we use this higherOrderFunction to repeatedly execute a specified function at fixed time intervals.

function sayHello() {
  console.log('Hello')
}
// This print "Hello" every 2 seconds
setInterval(sayHello, 2000)

# setTimeout

we use this higherOrderFunction to execute a specified function after a fixed time only once.

function sayHello() {
  console.log('Hello')
}
// This prints "Hello" only once after 2 seconds
setTimeout(sayHello, 2000) 

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