is()

Value equality check with semantics similar to Object.is, but treats Immutable Collections as values, equal if the second Collection includes equivalent values.

is(first, second): boolean

It's used throughout Immutable when checking for equality, including Map key equality and Set membership.

import { Map, is } from 'immutable';

const map1 = Map({ a: 1, b: 1, c: 1 });
const map2 = Map({ a: 1, b: 1, c: 1 });
assert.equal(map1 !== map2, true);
assert.equal(Object.is(map1, map2), false);
assert.equal(is(map1, map2), true);

is() compares primitive types like strings and numbers, Immutable.js collections like Map and List, but also any custom object which implements ValueObject by providing equals() and hashCode() methods.

Note: Unlike Object.is, Immutable.is assumes 0 and -0 are the same value, matching the behavior of ES6 Map key equality.