Why TypeScript's Structural Typing Still Trips Up Newcomers

Structural, not nominal

TypeScript compares types by shape, not by name. Two unrelated interfaces with the same members are interchangeable, which surprises developers coming from Java or C#.

interface Point { x: number; y: number }
interface Vector { x: number; y: number }

function log(p: Point) { console.log(p) }
log({ x: 1, y: 2 } satisfies Vector) // fine — same shape

This is by design: it matches how JavaScript objects actually behave at runtime, and it's what makes duck-typed APIs pleasant to use without a wall of boilerplate interfaces.

Where it bites

Excess property checks only apply to object literals assigned directly, not to variables. That inconsistency is the single most common source of "why did this compile?!" bug reports for TypeScript newcomers.