In JavaScript it's only the primitive objects (Number, String, Boolean, null, undefined) that are "by value". Everything else is "by reference". And when you wr

In JavaScript it's only the primitive objects (Number, String, Boolean, null, undefined) that are "by value". Everything else is "by reference". And when you write {foo:1} you create a new object. So it's logical that array.includes({foo:1}) is false. You can however point {foo:1} to a variable, and then use that variable as a reference, eg. var foo = {foo: 1}; array.push(foo); array.includes(foo);