JavaScript
Deep Clone
Copy a nested structure so nothing is shared — including the cases that break a naive recursion.
JavaScript · ES6 — Pratik Rai ·
#Objects#Recursion
Deep Equality
Compare two values structurally, because === only ever compares references.
By Pratik RaiMedium
Write deepEqual(a, b) returning whether the two values are structurally equal.
Two objects are equal when they have the same keys and every corresponding value is deeply equal; key order does not matter. Two arrays are equal when they have the same length and equal values in the same order. Primitives compare with ===, except that NaN equals NaN.
Examples
Input:
JSfile.javascript1deepEqual({ x: [1, 2] }, { x: [1, 2] });
Output:
true
Different objects, identical structure.
Constraints
- Values contain only objects, arrays, numbers, strings, booleans and null
Notes
- An array is never equal to an object, even one with matching numeric keys.
typeof nullis "object", so it needs its own guard.
Goal: Return true only when the two values match all the way down.
Sources
Frequently asked questions
- Why doesn't === work for comparing objects?
- Because it compares references, not contents. Two objects are `===` only when they are the same object, so `{a:1} === {a:1}` is false even though they look identical.
- How do you compare two objects by value?
- Recursively: same key count, then every key present in both with deeply equal values. Arrays additionally need the same length and matching order.
- What edge cases catch people out?
- `typeof null` is `"object"`, so `null` needs its own guard. An array and an object can share identical keys, so `Array.isArray` must match on both sides. And `NaN === NaN` is false, so equal `NaN`s need deciding explicitly.
- Is there a built-in for this?
- Not in the language. `Object.is` fixes `NaN` and `-0` but is still a shallow comparison. Test frameworks and utility libraries ship their own, and `JSON.stringify` comparison only works when key order happens to match.
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