Assertion Functions — Animate | AI Wisdom

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🎞 ANIMATE · Mode 3 of 5 · +100 XP— Assertion Functions
📋asserts
⚙️asserts is T
🔄assertIsDefined
✅assertNever
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asserts
asserts condition — narrow type based on truthiness💡 Analogy
Like a quality inspector — if they sign off, you know the item passed inspection.
asserts condition tells TypeScript that if the function returns normally (no throw), condition is truthy. Any code after assert(x) knows x is truthy.
Console Output
> assert(user !== null)
> TypeScript: user is not null after this line
> assert(false) → throws Error("Assertion failed")
●assertion-functions.ts
assertsL2–3
1// assert asserts is T — throw if condition false
2function assert(condition: any, msg?: string): asserts condition {
3  if (!condition) throw new Error(msg ?? 'Assertion failed');
4}
5 
6// asserts x is T — type narrowing via assertion
7function assertIsString(val: unknown): asserts val is string {
8  if (typeof val !== 'string') {
9    throw new TypeError(`Expected string, got ${typeof val}`);
10  }
11}
12 
13// Usage — type narrows after the assertion call
14function processInput(input: unknown) {
15  assertIsString(input);
16  // input: string here — TypeScript narrowed it!
17  console.log(input.toUpperCase()); // safe
18}
19 
20// assertIsDefined — common utility
21function assertIsDefined<T>(val: T): asserts val is NonNullable<T> {
22  if (val === undefined || val === null) {
23    throw new Error('Expected defined value');
24  }
25}
26 
27// Combined use
28function renderUser(user: User | null) {
29  assertIsDefined(user);
30  // user: User — null ruled out
31  console.log(user.name); // safe, no ?. needed
32}
33 
34// assertNever — exhaustive checking
35function assertNever(x: never): never {
36  throw new Error(`Unhandled case: ${JSON.stringify(x)}`);
37}
38 
39interface User { name: string; }
39 lines
Timeline — 4 Steps
asserts
asserts condition — narrow type based on truthiness💡 Analogy
Like a quality inspector — if they sign off, you know the item passed inspection.
asserts condition tells TypeScript that if the function returns normally (no throw), condition is truthy. Any code after assert(x) knows x is truthy.
Console Output
> assert(user !== null)
> TypeScript: user is not null after this line
> assert(false) → throws Error("Assertion failed")

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Discussion

asserts condition — narrow type based on truthiness

💡 Analogy

Like a quality inspector — if they sign off, you know the item passed inspection.

asserts condition tells TypeScript that if the function returns normally (no throw), condition is truthy. Any code after assert(x) knows x is truthy.

Console Output

> assert(user !== null)
> TypeScript: user is not null after this line
> assert(false) → throws Error("Assertion failed")

asserts condition — narrow type based on truthiness

💡 Analogy

Like a quality inspector — if they sign off, you know the item passed inspection.

asserts condition tells TypeScript that if the function returns normally (no throw), condition is truthy. Any code after assert(x) knows x is truthy.

Console Output

> assert(user !== null)
> TypeScript: user is not null after this line
> assert(false) → throws Error("Assertion failed")