Nanfeng

Notes on software development, code, and curious ideas

TypeScript Object Assignment and Equality: References, Shallow and Deep Copy

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Understand TypeScript object references, assignment, shallow and deep copying, strict equality, structural comparison, and prototype effects with examples.

Objects and arrays in JavaScript and TypeScript are reference values. Assigning one variable to another with = makes both variables refer to the same object:

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let a = { value: 1 };
let b = a;
console.log(a === b); // true
b.value = 3;
console.log(a.value); // 3

A shallow copy duplicates only the first level. Nested objects remain shared. For a simple serializable object, one deep-copy option is:

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const copy = JSON.parse(JSON.stringify(original));

This does not preserve functions, dates, special types, or circular references. Modern environments can use structuredClone; Lodash also provides cloneDeep:

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import _ from 'lodash';
const copy = _.cloneDeep(original);

Two separate objects with identical properties are not equal under === because their references differ:

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const a = { x: 1 };
const b = { x: 1 };
console.log(a === b); // false
console.log(_.isEqual(a, b)); // true

Class instances can also differ in their prototype chains. Decide whether your operation requires identity comparison, shallow copying, deep copying, or structural equality, and choose the corresponding mechanism explicitly.

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