JavaScript Declarative and Functional Principles

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Imperative Vs. Declarative Code

There are two fundamentally different ways of writing code (paradigms):

Imperative

• Programmer explains “HOW to do things”.
• We explain the computer every single step it has to follow to achieve a result.
• Example: Step-by-step recipe of a cake.

Declarative

• Programmer tells “WHAT to do”
• We simply describe the way the computer should achieve the result
• The HOW (step-by-step instructions) gets abstracted away.
• Example: Description of a cake.

Functional Programming

• Declarative programming paradigm.
• Based on the idea of writing software by combining many pure functions, avoiding side effects and mutating data.
• Side effect: Modification (mutation) of any data outside of the function (mutating external variables, logging to console, writing to DOM, etc.).
• Pure function: Function without side effects. Does not depend on external variables. Given the same inputs, always returns the same outputs.
• Immutability: State (data) is never modified! Instead, state is copied and the copy is mutated and returned.
• Examples: React - Redux.

Functional Programming Techniques

• Try to avoid data mutations (e.g. using Object.freeze()).
• Use built-in methods that don’t produce side effects.
• Do data transformations with methods such as .map(), .filter() and .reduce().
• Try to avoid side effects in functions: this is of course not always possible!

Declarative Syntax

• Use Array and Object Destructuring.
• Use the Spread Operator (...).
• Use the ternary (conditional) operator.
• Use template literals.