JavaScript Functions are First-Class Citizens

Summary: in this tutorial, you’ll learn that JavaScript functions are first-class citizens. This means that you can store functions in variables, pass them to other functions as arguments, and return them from other functions as values.

Storing functions in variables

Functions are first-class citizens in JavaScript. In other words, you can treat functions like values of other types.

The following defines the add() function and assigns the function name to the variable sum:

function add(a, b) {
    return a + b;
}

let sum = add;Code language: JavaScript (javascript)

In the assignment statement, we don’t include the opening and closing parentheses () at the end of the add function name. Additionally, we don’t execute but reference the function only.

By doing this, we can have two ways to execute the function. For example, we can call it normally as follows:

let result = add(10, 20);Code language: JavaScript (javascript)

Alternatively, we can call the add() function via the sum variable like this:

let result = sum(10,20);Code language: JavaScript (javascript)

Passing a function to another function

Since functions are values, you can pass them as arguments into other functions.

For example, the following declares the average() function that takes three arguments. The third one is a function:

function average(a, b, fn) {
    return fn(a, b) / 2;
}Code language: JavaScript (javascript)

Now, you can pass the sum function to the average() function as follows:

let result = average(10, 20, sum);Code language: JavaScript (javascript)

Put it all together:

function add(a, b) {
    return a + b;
}

let sum = add;

function average(a, b, fn) {
    return fn(a, b) / 2;
}

let result = average(10, 20, sum);

console.log(result);Code language: JavaScript (javascript)

Output:

15Code language: JavaScript (javascript)

Returning functions from functions

Since functions are values, you can return a function from another function.

The following compareBy() function returns a function that compares two objects by a property:

function compareBy(propertyName) {
  return function (a, b) {
    let x = a[propertyName],
      y = b[propertyName];

    if (x > y) {
      return 1;
    } else if (x < y) {
      return -1;
    } else {
      return 0;
    }
  };
}Code language: JavaScript (javascript)

Note that a[propertyName] returns the value of the propertyName of the a object. It’s equivalent to a.propertyName. However, if the propertyName contains a space like 'Discount Price', you need to use the square bracket notation to access it.

Suppose that you have an array of product objects where each product object has two properties: name and price.

let products = [
    {name: 'iPhone', price: 900},
    {name: 'Samsung Galaxy', price: 850},
    {name: 'Sony Xperia', price: 700}
];Code language: JavaScript (javascript)

You can sort an array by calling the sort() method. The sort() method accepts a function that compares two elements of the array as an argument.

For example, you can sort the product objects based on the name by passing a function returned from the compareBy() function as follows:

console.log('Products sorted by name:');
products.sort(compareBy('name'));

console.table(products);Code language: JavaScript (javascript)

Output:

Products sorted by name:
┌─────────┬──────────────────┬───────┐
│ (index) │       name       │ price │
├─────────┼──────────────────┼───────┤
│    0    │ 'Samsung Galaxy' │  850  │
│    1    │  'Sony Xperia'   │  700  │
│    2    │     'iPhone'     │  900  │
└─────────┴──────────────────┴───────┘Code language: plaintext (plaintext)

Similarly, you can sort the product objects by price:

// sort products by prices
console.log('Products sorted by price:');
products.sort(compareBy('price'));
console.table(products);Code language: JavaScript (javascript)

Output:

Products sorted by price:
┌─────────┬──────────────────┬───────┐
│ (index) │       name       │ price │
├─────────┼──────────────────┼───────┤
│    0    │  'Sony Xperia'   │  700  │
│    1    │ 'Samsung Galaxy' │  850  │
│    2    │     'iPhone'     │  900  │
└─────────┴──────────────────┴───────┘Code language: JavaScript (javascript)

Put it all together.

function compareBy(propertyName) {
  return function (a, b) {
    let x = a[propertyName],
      y = b[propertyName];

    if (x > y) {
      return 1;
    } else if (x < y) {
      return -1;
    } else {
      return 0;
    }
  };
}
let products = [
  { name: 'iPhone', price: 900 },
  { name: 'Samsung Galaxy', price: 850 },
  { name: 'Sony Xperia', price: 700 },
];

// sort products by name
console.log('Products sorted by name:');
products.sort(compareBy('name'));

console.table(products);

// sort products by price
console.log('Products sorted by price:');
products.sort(compareBy('price'));
console.table(products);Code language: JavaScript (javascript)

More JavaScript Functions are First-Class Citizens example

The following example defines two functions that convert a length in centimeters to inches and vice versa:

function cmToIn(length) {
    return length / 2.54;
}

function inToCm(length) {
    return length * 2.54;
}Code language: JavaScript (javascript)

The following convert() function has two parameters. The first parameter is a function and the second one is the length that will be converted based on the first argument:

function convert(fn, length) {
    return fn(length);
}Code language: JavaScript (javascript)

To convert cm to in, you can call the convert() function and pass the cmToIn function into the convert() function as the first argument:

let inches = convert(cmToIn, 10);
console.log(inches);Code language: JavaScript (javascript)

Output:

3.937007874015748Code language: JavaScript (javascript)

Similarly, to convert a length from inches to centimeters, you can pass the inToCm function into the convert() function, like this:

let cm = convert(inToCm, 10);
console.log(cm);Code language: JavaScript (javascript)

Output:

25.4Code language: JavaScript (javascript)

Put it all together.

function cmToIn(length) {
  return length / 2.54;
}

function inToCm(length) {
  return length * 2.54;
}

function convert(fn, length) {
  return fn(length);
}

let inches = convert(cmToIn, 10);
console.log(inches);

let cm = convert(inToCm, 10);
console.log(cm);Code language: JavaScript (javascript)

Output:

3.937007874015748
25.4Code language: JavaScript (javascript)

Summary

• Functions are first-class citizens in JavaScript.
• You can pass functions to other functions as arguments, return them from other functions as values, and store them in variables.

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