What is prototypal inheritance?

Quick answer

Objects delegate to other objects via a prototype link; property lookups walk the prototype chain until found or null.

In detail

Each object has an internal [[Prototype]] pointing to another object. Missing properties are looked up along the chain. Shared methods live once on a prototype while many instances delegate to it — memory-efficient and the basis of how `class` works under the hood.

const arr = [1, 2, 3];
arr.map;  // found on Array.prototype, not on arr
Object.getPrototypeOf(arr) === Array.prototype; // true

Why interviewers ask this: Separates people who memorised classes from those who get the model.

Common follow-up questions

→What's the difference between __proto__ and prototype?

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