Thinking in Expressions

October 29, 2024

What Are Expressions?

Imperative programs are built from statements, and read top to bottom.

let numbers = [1, 2, 3];
let sum = 0;
for (let i = 0; i < numbers.length; i++) {
  sum += numbers[i];
}

Functional programs are composed of expressions, and read inside out.

Here’s the same program written in a functional style:

[1, 2, 3].reduce((a, b) => a + b, 0)

These expressions are atomic:

1 2 3 a b 0

We compose bigger expressions from smaller ones:

Zoom out a little more:

Zoom out all the way:

[1, 2, 3].reduce((a, b) => a + b, 0)

💡 In functional programming, every value is an expression, including the program itself.

FP offers a simpler way of looking at the world:

Every value is an expression
- Strings, numbers, and booleans are expressions
- Arrays and objects are expressions
- Functions are expressions
Any expression can be replaced with its simplest value without changing the program:
- 1 + 2 can be replaced by 3 and vice versa
- 5 * 3 can be replaced by 15 and vice versa
- Math.sqrt(16) can be replaced by 4 and vice versa
- [1, 2, 3].reduce((a, b) => a + b, 0) can be replaced by 6 and vice versa

For example, let’s extract [1, 2, 3] into a variable:

let numbers = [1, 2, 3]
numbers.reduce((a, b) => a + b, 0)

Extract (a, b) => a + b into a variable:

let numbers = [1, 2, 3]
let add = (a, b) => a + b
numbers.reduce(add, 0)

Wrap it into a reusable function:

let add = (a, b) => a + b
let sum = (numbers) => numbers.reduce(add, 0)
sum([1, 2, 3])