name: coding-patterns description: Use any time you are writing TypeScript code.

Coding Patterns

General

is and assert

Use the @sindresorhus/is library for simple runtime type checks & assertions.

The is utility contains methods that return true if the given input matches the indicated type, and false otherwise:

const unknownValue: unknown = 'hello world';

// Correct
is.truthy(unknownValue); // true
is.string(unknownValue); // true
is.number(unknownValue); // false
is.boolean(unknownValue); // false
is.nullOrUndefined(unknownValue); // false
is.nonEmptyStringAndNotWhitespace(unknownValue); // true

// Incorrect
Boolean(unknownValue);
!!unknownValue;
typeof unknownValue === 'string';

The assert utility contains all the same methods as is, however instead of returning a boolean value, it throws an error if the expectation fails. Use this instead of type assertions, and instead of conditionals that check the truthiness of a value:

const unknownValue: unknown = true;

// Correct
assert.truthy(unknownValue); // TS knows `unknownValue` is truthy now

// Incorrect
if (!unknownValue) {
  throw new Error('Expected unknown value to be truthy');
}

Lodash

Use named imports from the lodash/es module for functional patterns.

Generally, the following lodash utilities are recommended to be used instead of manually implementing them with loops or array map/reduce methods:

• groupBy
• keyBy
• countBy
• partition
• compact

When possible, use the shorthand syntax:

// Correct
const objectsById = groupBy(objects, '_id');
const [valid, invalid] = partition(results, 'isValid');

// Incorrect
const objectsById = groupBy(objects, o => o._id);
const [valid, invalid] = partition(results, result => result.isValid);

Frontend Patterns

React

Do not use useCallback or useMemo. Instead, functions and computed values in a component can be directly assigned to local variables. This project uses the React Compiler with React 19, so memoization of local variables is automatic.

TanStack Router

Client-side routes are managed by TanStack Router, and are defined in packages/client/src/routes.