Mutable values are values that can be changed without an entirely new value. In JavaScript objects and arrays are mutable by default. As already said in a previous post about Immutable Primitive Values, there is a fundamental difference in JavaScript between primitive values (undefined, null, booleans, numbers, and strings) and objects (including arrays and functions).

First, objects are mutable:

let obj = { x: 1 }
o.x = 2              // obj = { x: 2 }
o.y = 3              // obj = { x: 2, y: 3 }

Second, objects are not compared by value, two objects are not equal even if they have the exact same values, this is true for arrays too:

let o = { x: 1 }, p = { x: 1 }
o === p                         // false
let a = [], b = []
a === b                         // false

JavaScript objects and arrays are compared by reference, two objects values are the same if and only if they refer to the same underlying object:

let a = []
let b = a     // b refers to a
b[0] = 1
a[0]          // 1 - changes are also visible in the array a
a === b       // true

Thanks for Reading!