Higher Order

higher order function¶

A function that takes a function as an argument or returns a function as a result.

Consider this code block:

let nums = [1,2,3,4,5];

function printSquare(number, index) {
  console.log(number * number);
};

nums.forEach = function() {
  for (let i = 0 ; i < this.length ; i++) {
    printSquare(this[i], i);
  }
};

nums.forEach();

The above function (nums.forEach) will use the function printSquare to print out the squared value of the current value being iterated over in the array.

This forEach method is not very useful as it is bound to use one specific function implementation (printSquare).

Dynamically Altering Behavior by Providing Functions as Arguments¶

In the following example, instead of always using the same function, we will use a function provided when the forEach method is invoked. * This will make our method much more dynamic and allow the end user to specify how it be implemented.

let nums = [1,2,3,4,5];

nums.forEach = function(providedFunc) {
  for (let i = 0 ; i < this.length ; i++) {
    providedFunc(this[i], i , this);
  }
};

function printSquare (number, index, array) {
  console.log(number * number);
}

function printSum (number, index, array) {
  console.log(number + number);
}

function isEven (number, index, array) {
  console.log(number % 2 === 0);
}

nums.forEach(printSquare);
nums.forEach(printSum);
nums.forEach(isEven);

We have never seen behavior like this before. * Because functions are considered 'values', no different than a number, string, or object, they can be passed to functions and used in a similar fashion.

### Skill Drill 1. When we define a function and print the value of the assigned name, what do we get?

```javascript
function example1 (){
  console.log("example1");
}

console.log(example1);  

// or

let example2 = function(){
  console.log("example2");
}
console.log(example2);
```

This would have given us an error in Java. But in JavaScript, even an uninvoked function still has a value, its assigned function. We can store this value into other variables if we want to.
```
  let example2 = function(){
    console.log("example2");
  }

  let test = example2;
  console.log(test);
  test();
```
This is essentially what happens when we pass a function as an argument to another function. The provided function is assigned to the function's parameter, which we can use from within the function.

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