TypeScript Academy · Lesson

Default Values in Destructuring

Combine defaults with type annotations safely.

Default Values in Destructuring is a free TypeScript Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the TypeScript Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Filling in the Blanks

Destructuring defaults supply a fallback when a property or element is missing or undefined. Combined with TypeScript's inference, they make optional configuration ergonomic and safe.

A Default in Object Destructuring

Write = value after a binding to give it a fallback. If the property is absent or undefined, the default is used instead.

const opts: { count?: number } = {}
const { count = 0 } = opts
console.log(count) // 0

Defaults With Optional Properties

Defaults pair naturally with optional (?) properties. The default removes the undefined case, so the resulting variable is the non-optional type.

interface Opts { retries?: number }
function connect(opts: Opts) {
  const { retries = 3 } = opts
  // retries is number, not number | undefined
  return retries * 2
}
console.log(connect({}))

Defaults and Inferred Types

If you do not annotate, TypeScript infers the variable's type from the default value. Here 0 makes count a number.

const settings: { count?: number; label?: string } = { label: "hi" }
const { count = 0, label = "none" } = settings
console.log(count, label) // 0 hi

Only undefined Triggers the Default

A subtle point: defaults apply only for undefined, never for other falsy values like 0, "", or null. Those pass through unchanged.

const { x = 10 } = { x: 0 }
console.log(x) // 0, not 10 — 0 is not undefined
const { y = 10 } = { y: undefined as number | undefined }
console.log(y) // 10

Defaults in Function Parameters

Destructured parameter defaults make optional options painless. Each field can have its own fallback right in the signature.

function draw({ x = 0, y = 0 }: { x?: number; y?: number }) {
  return x + "," + y
}
console.log(draw({ x: 5 })) // 5,0

Defaulting the Whole Parameter

To let callers omit the argument object entirely, give the parameter itself a default of {}. Without this, calling with no argument would throw.

function draw({ x = 0, y = 0 }: { x?: number; y?: number } = {}) {
  return x + "," + y
}
console.log(draw()) // 0,0

Defaults With Renaming

You can rename and default in one binding: rename first, then add the default after the new name.

const { count: total = 0 } = { } as { count?: number }
console.log(total) // 0

Computed Defaults

A default can be any expression, even one referencing earlier bindings. It is evaluated lazily — only when the value is actually missing.

function make({ width = 100, height = width }: { width?: number; height?: number } = {}) {
  return width + "x" + height
}
console.log(make({ width: 50 })) // 50x50

Array Defaults Recap

Defaults work in array destructuring too, applying per position when an element is missing or undefined.

const [a = 1, b = 2, c = 3] = [10, undefined]
console.log(a, b, c) // 10 2 3

When Defaults Shine

Defaults are ideal for options objects: callers specify only what they care about, and the function fills sensible fallbacks. They also remove undefined from inferred types, simplifying the body.

interface Config { debug?: boolean; level?: number }
const init = ({ debug = false, level = 1 }: Config = {}) => debug ? level : 0
console.log(init({ debug: true, level: 5 }))

Quick Check

Test your understanding of destructuring defaults.

Recap

Destructuring defaults (= fallback) apply only when a value is missing or undefined — never for other falsy values. They remove undefined from optional-property types, can be inferred or annotated, support renaming, and may reference earlier bindings. For options objects, default the parameter to {} so callers can omit it entirely.

Frequently asked questions

Is the “Default Values in Destructuring” lesson free?

Yes — the full text of “Default Values in Destructuring” is free to read here on the web, and the TypeScript Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the TypeScript Academy course, upgrade to CoddyKit PRO.

What will I learn in “Default Values in Destructuring”?

Combine defaults with type annotations safely. You practise TypeScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start TypeScript Academy?

No prior experience is required. TypeScript Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Default Values in Destructuring” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this TypeScript Academy lesson?

Yes. Every TypeScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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