First-class Functions

An important thing to remember about functions in JavaScript is: functions are values.

This means that functions are treated as first-class citizens, just like numbers and objects. They have all the properties we expect of other objects. Check out this medium blog post for a more in-depth explanation.

Let's look at this example:

// A named function
function triple (x) {
  return x * 3
}

// An anonymous function
function (x) {
  return x * 3
}

// An anonymous function assigned to a variable
const triple = function (x) {
  return x * 3
}

// An anonymous fat arrow function assigned to a variable
const triple = (x) => {
  return x * 3
}

Because the last example uses a variable, you can do some cool things like this:

const engorgio = triple
const value = engorgio(30) // value is 90

Both triple and engorgio refer to the same function. But the fun does not stop there! We can also pass functions into other functions as arguments.

A good example of this is the filter function, probably the most basic and useful higher order function. It's a function that operates on an array, and accepts another function as an argument that it uses to return a new filtered version of the array.

// Original array
const names = ['Alice', 'Bob', 'Charlie', 'David', 'Eva'];

// Use filter to get names with length greater than 4
const longNames = names.filter(function(name) {
  return name.length > 4;
});

console.log(longNames); // Output: ['Alice', 'Charlie']

So, to recap:

• We can store functions in variables
• We can store functions as properties of other objects
• Functions can have properties
• We can pass functions as arguments into other functions