top-down-typescript

Top-Down TypeScript Coding Style

General Rules

• Dead (unused) code SHOULD NOT be kept (YAGNI principle).
• Do not write multiple consecutive blank lines.
• Changes to linter rules MUST be discussed before being implemented.
• Code SHOULD NOT contain commented-out code, unless a comment explains why.

Code Organization

Usage comes first, implementation after. Exception: with inheritance — when an interface extends another, write the parent first.

• Order code top-down: each file reads as a story, from entry point to leaves. The reader meets the highest-level thing first, then drills down into its dependencies:

  // 1. Imports
  import { ... } from "...";
  
  // 2. Module-level constants and variables (exported first, then internal)
  export const PUBLIC_CONST = ...;
  const INTERNAL_CONST = ...;
  
  // 3. Shared types — main type first, then types it references
  export interface MainType {
    detail: DetailType;
  }
  export interface DetailType { ... }
  
  // 4. Entry-point (exported) function
  export function doThing() {
    stepOne();
    stepTwo();
  }
  
  // 5. Internal functions called by the entry point, in call order
  function stepOne() {
    stepOneHelper();
  }
  function stepOneHelper() { ... }
  
  function stepTwo() { ... }
  

• Module-level constants and variables (const, let, var value declarations at the top of the file — both exported and internal) MUST be placed immediately after imports, before any type definitions, functions, or classes. The reader sees them first and treats them as the file's configuration surface.

• Functions: write the caller first, then the functions it calls, recursively. A helper appears just below its caller, not grouped at the bottom of the file. If a helper is called by several siblings, place it after its first caller.

• Types: write the main (top-level) type first, then the types it references, recursively. Same top-down rule as functions.

• Types attached to a single function (or class, or other declaration) — i.e. used only in that one signature, like a MyComponentProps interface used only by MyComponent — must be placed immediately before that declaration, not in the top type block.

• Exports are not a sorting criterion on their own: a function being exported does not pull it to the top — its position is determined by who calls it. The entry points of a file are usually exported, which is why they tend to appear first, but that is a consequence of the top-down rule, not the rule itself.

Code Quality Standards

• Strive for elegant solutions from the first implementation
• Avoid redundant operations, especially expensive ones like image conversion
• Avoid duplicated code and logic
• Pass previously calculated values between functions instead of recalculating
• Use early returns to simplify code flow when possible
• For code that leaves the current flow (throw, return, continue, break), when it fits on one line, write it on one line (e.g., if (!condition) return false; instead of multi-line format)
• Use function and variable names that clearly convey intent, reducing the need for comments
• Keep functions small with a single responsibility
• Avoid any; take the time to find the proper type. If you fail to find one, always insert a /* FIXME */ after the any. For example: let myVariable: any /* FIXME */;.
• Export only functions (or variables, classes) that are imported from elsewhere. By default, do not export.
• When an interface is used in the signature of an exported function or component, that interface must also be exported.

Imports

• Always use ESM import syntax (e.g., import { X } from "y.js" instead of require).
• Avoid circular imports between modules.

TypeScript, JavaScript

• Never use enum and namespace.
• Prefer const over let.
• Prefer undefined over null.
• Prefer ?? over ||.
• Prefer ++i and --i over i++ and i--.
• Prefer new Error() over Error().
• At the top level, prefer the function and class declarative syntax over creating them as constants.
• Keep an empty line between top-level functions, classes, interfaces.
• Implementation of a getter or setter (EcmaScript 5 syntax) must never throw exceptions.
• Prefer interface declarations over type aliases.
• Prefer a single capital letter for generics parameters, such as T, K, etc.
• Do not differentiate between an absent property and a property with an undefined value.
• Use camelCase for string literal values in TypeScript union types (e.g., "normal" | "gracefulShutdown" | "backupMode" instead of "normal" | "graceful-shutdown" | "backup-mode").
• Never use an empty string as a default value unless you really mean an empty string. If a variable might not have a value, use undefined or throw an error if the absence of value indicates a problem.
• The existence of string, number, boolean values (and identifiers when they are string or number) must NEVER be tested by coercing to boolean. Use explicit comparisons with undefined or null.
• Existence checks for objects and arrays MAY use boolean coercion.
• Never explicitly assign or return undefined when it is the default value. Use return; instead of return undefined; and let myVariable; instead of let myVariable = undefined;. Explicitly passing undefined is fine when intentionally setting a value.
• Avoid as any or any kind of type assertion. Always make the effort to find the proper type. Exception: when the type is incorrect or truly unknown — justify with an inline comment.
• Never re-export, except from the package's index file.
• Avoid inline import("some-package-or-module").SomeType; prefer direct imports at the top of the file.
• Avoid inline await import("some-package-or-module"); prefer static imports at the top of the file. Exception: when there is a valid reason — justify with an inline comment.

OOP

• Prefer factory functions over classes.
• Prefer writing functions with a context object instead of a class.
• Avoid class inheritance, except in the context of a framework that requires it.

Adding a package dependency

Before adding a dependency or dev-dependency, search the codebase first and reuse the version already in use. If not found, install the latest version using the default install command.

Commit, PR/MR, and changeset messages

Never add AI attribution — Co-Authored-By: …, "Generated with …", or similar — to a commit, PR, MR, or changeset message.

Changeset messages

Write for someone already using the project who wants to know what changed, in a few words. Mention only what is actionable for them; skip the why and the internal details. Always a single short paragraph.

• New feature: name it in a few words.
• Extends a feature: title it if obvious, otherwise "Improved the {X} feature."
• Nothing actionable (for example documentation or refactoring): stay succinct, like "Improved documentation about {topic}."

When a version mixes actionable and non-actionable changes (for example a big refactoring plus a small feature), mention only the actionable one. Mention non-actionable changes only when there is nothing else for the user.

Version bumps

Never bump a package from 0.x.x to 1.0.0 unless explicitly instructed. Breaking changes are accepted while the major version is 0.

Improving code quality

SRP - Single Responsibility Principle

Think of it as narrative decomposition: the caller reads like a paragraph that names what happens; each helper expands one sentence of that paragraph.

For example, this code:

export function myFunction() {
  // Check something
  // ... 20 lines ...

  // Do something
  // ... 20 lines ...
}

… should be refactored in:

export function myFunction() {
  checkSomething();
  doSomething();
}

function checkSomething() {
  // ... 20 lines ...
}

function doSomething() {
  // ... 20 lines ...
}

Guidelines:

• Always write the sub-function after the caller function.
• Do not export a function unless it is imported from outside the source file.
• Apply the Single Responsibility Principle when dividing code: one function for one concern.
• Avoid exceeding the height of one screen (~50 lines) for function implementations.
• Keep code clean and self-explanatory rather than adding explanatory comments.

DRY - Don't Repeat Yourself

Each time you see duplicated logic, take the time to refactor it into a reusable function.

YAGNI - You Aren't Gonna Need It

Do not keep unused code such as variables, functions, implementations, etc.

Warning signs

• Fallbacks to empty string or zero rarely have a good reason: review every ?? "" and ?? 0 and confirm it is intentional. Otherwise, understand the typing and find an elegant fix.
  • Note: a valid use case for ?? "" is when the UI requires an empty string.
• Type assertions (as SomeType) are often a sign of misunderstood typing.
• Avoid any and find the proper type. When any is truly needed, add a comment explaining why.
• Do not use ReturnType<T> or Parameters<T> if you can import the actual type.
• Do not use SomeType["someMemberName"] if you can import the actual type.

Remove Unnecessary Comments

• About comments: the fewer the better. Comments are read by skilled developers. Each comment must be sharp, concise, straight to the point. Each word must be carefully weighted and chosen.
• Remove comments that are redundant with the code itself.
• Only keep inline comments that document hacks, TODOs, or exceptional situations, or when the code's purpose isn't obvious from its structure.
• Do not use comments as annotations for justifying the task you are currently working on.

The following comment must be removed:

// Create a new task
createANewTask();

Other examples of inline comments that are obvious and must be removed:

// Validate that there is a file
if (!file) throw new ApiError(400);

// Validate and parse the request body according to the expected schema
const validated = UploadBodyAT.assert(body);

JSDoc comments should only be used when they add meaningful information. Example of a comment that must be entirely removed because everything is obvious:

/**
 * This function adds two numbers
 * @param a - The first number
 * @param b - The second number
 * @returns The sum of the two numbers
 */
function addTwoNumbers(a: number, b: number) {
  return a + b;
}