Declarative and Imperative Programming

This excerpt is taken from Eric Elliot article about functional programming.

Functional programming is a declarative paradigm, meaning that the program logic is expressed without explicitly describing the flow control.

Imperative programs spend lines of code describing the specific steps used to achieve the desired results — the flow control: How to do things.

Declarative programs abstract the flow control process, and instead spend lines of code describing the data flow: What to do. The how gets abstracted away.

For example, this imperative mapping takes an array of numbers and returns a new array with each number multiplied by 2:

const doubleMap = numbers => {
  const doubled = [];
  for (let i = 0; i < numbers.length; i++) {
    doubled.push(numbers[i] * 2);
  }
  return doubled;
};

console.log(doubleMap([2, 3, 4])); // [4, 6, 8]

This declarative mapping does the same thing, but abstracts the flow control away using the functional Array.prototype.map() utility, which allows you to more clearly express the flow of data:

const doubleMap = numbers => numbers.map(n => n * 2);

console.log(doubleMap([2, 3, 4])); // [4, 6, 8]

Imperative code frequently utilizes statements. A statement is a piece of code which performs some action. Examples of commonly used statements include for, if, switch, throw, etc…

Declarative code relies more on expressions. An expression is a piece of code which evaluates to some value. Expressions are usually some combination of function calls, values, and operators which are evaluated to produce the resulting value.