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JavaScript interview question

Deep Equal

medium25 min

Implement deepEqual(left, right) to compare values by their contents rather than only their references. Walk nested arrays and plain objects and decide whether both structures contain the same values.

This exercise focuses on primitives, arrays, and plain objects. Arrays can be sparse, so an absent slot is different from a present slot containing undefined.

Requirements

Export deepEqual as both a named and default export. Return a boolean.
Inputs contain only undefined, null, booleans, numbers, strings, arrays, and plain objects, including null-prototype objects. Circular structures and other object types are outside scope.
Compare primitives without coercion. NaN equals NaN for this utility; positive and negative zero are equal.
Compare arrays in order, including their length and the presence of own indices. A hole is different from an explicit undefined element. Inputs have no custom array properties.
Compare plain objects using their own enumerable string keys. Key order and the object prototype do not affect equality. Ignore inherited, symbol, and non-enumerable properties.
An array and a plain object are different types even if their keys look alike. Missing object keys differ from keys whose value is undefined.
Do not mutate either input or use JSON serialization as the comparison algorithm.

Examples

Example 1

Input: deepEqual({ scores: [2, { passed: true }] }, { scores: [2, { passed: true }] });
Output: true

Example 2

Input: deepEqual({ value: undefined }, {});
Output: false

Example 3

Input: deepEqual(new Array(1), [undefined]);
Output: false

Example 4

Input: deepEqual(NaN, NaN);
Output: true

The JavaScript and TypeScript variants use the same API and behavior.

What the tests check

Compares primitive values without coercion
Handles NaN, infinities, and signed zero
Compares nested arrays and objects
Ignores object key insertion order
Distinguishes missing keys from undefined values
Checks array order and length
Preserves sparse array structure
Does not confuse arrays with plain objects
Ignores inherited and non-enumerable properties
Handles null-prototype objects and special property names
Does not mutate frozen inputs

References

Object.keys()Number.isNaN()

RecursionObjectsArraysEquality

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