How does a JavaScript `Set` determine whether two values are duplicates?

A Set allows each value only once according to SameValueZero equality. For primitive values, adding the same value again does not add a second entry. Objects work differently from a comparison of their contents: a Set checks whether the object is the same reference. So two objects that look alike can both be entries. [S1]

That distinction matters when a value is simple, like a number, versus when it is an object created separately elsewhere in your code.

Here is a small example you can run:

const primitives = new Set(["tea", "tea", 7, 7]);
console.log(primitives.size); // 2

const item = { name: "tea" };
const sameReference = new Set([item, item]);
console.log(sameReference.size); // 1

const lookalikes = new Set([
  { name: "tea" },
  { name: "tea" }
]);
console.log(lookalikes.size); // 2

The primitive set has one entry for "tea" and one for 7. In the second case, both entries refer to the same object. In the last case, each object literal creates a separate object, so matching name properties do not make them duplicates. [S1]

Because object uniqueness in a Set is based on identity: whether the references point to the same object, rather than whether their properties match. In the example, item is one object reference used twice. The two object literals are separate references, even though both have a name property with the same value. [S1]

A useful way to check what your code is doing is to look at where each object is created. Reusing a variable reuses its reference; creating another object literal creates another object. If your goal is to treat matching properties as duplicates, define that comparison explicitly rather than expecting the Set to compare object contents.

Check whether the records are separate objects and whether you are adding the records themselves or their IDs. Separate record objects remain distinct Set entries even when their id properties match. If uniqueness should be based on IDs, one direct option is to build a set from those primitive IDs:

const records = [
  { id: "a1", label: "First" },
  { id: "a1", label: "Second" }
];

const ids = new Set(records.map(record => record.id));
console.log(ids.size); // 1

This example deduplicates the ID values; it does not select or combine a single record for you. Decide separately which record to keep if you need a deduplicated record list. The Set rule does not determine your application’s preferred handling of records that share an ID but differ elsewhere.