typescript

TypeScript / JavaScript Language Idioms

Work with the language. Embrace its asynchronous nature, its structural type system, and its functional roots. Fight the urge to write Java-in-TypeScript.

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Core Philosophy

• Immutability by default — const everything, spread to copy, never mutate in place
• Type safety without ceremony — let inference work, annotate at boundaries
• Composition over inheritance — functions, modules, and interfaces beat class hierarchies
• Explicit over implicit — no coercion tricks, no hidden state, no magic

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References and Variables

The Rules

Keyword	Usage	Rationale
const	Default for all bindings	Prevents reassignment, signals intent
let	Only when reassignment is unavoidable	Loops, accumulators, state machines
var	Prohibited	Function-scoped, hoists, leaks out of blocks

Destructuring and Spread

Use destructuring to extract what you need. Use spread to copy without mutation:

// ✅ Destructure at the call site
const { name, age } = getUser();

// ✅ Shallow-copy with spread — never mutate the original
const updated = { ...config, timeout: 5000 };
const extended = [...items, newItem];

// ❌ Mutating the original
config.timeout = 5000;
items.push(newItem);

Use Object.hasOwn(obj, key) instead of obj.hasOwnProperty(key) — the latter can be shadowed.

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Functions

Arrow Functions

Use arrow functions for callbacks and short expressions. They preserve lexical this:

// ✅ Arrow for callbacks
const sorted = items.sort((a, b) => a.name.localeCompare(b.name));

// ✅ Arrow for inline transforms
const names = users.map((u) => u.name);

// ❌ Legacy — don't use `function` for callbacks
const names = users.map(function (u) {
  return u.name;
});