The ES6 JavaScript const keyword prevents re-assignment of a reference, but it does not actually prevent data modification of any object being referenced.
Of course, a primitive value declared const cannot be changed:
> const pi = 3.1415926 > pi = 3 TypeError: Assignment to constant variable.
However, objects declared with const can still have their properties changed:
const myObject = {
a: 5,
b: 10,
c: 15
}
myObject.b = -1 // no error
console.dir(myObject)
Result:
{ a: 5, b: -1, c: 15 }
Similarly, arrays declared with const in ES6 can be edited! They simply cannot be re-assigned.
const arr = [1, 2, 3, 4, 5] a[1] = -1 // this is fine console.dir(arr)
Result:
[1, -1, 3, 4, 5]
Using methods to manipulate the const array is still allowed:
const arr = [1, 2, 3, 4, 5] arr.pop() // allowed console.dir(arr)
Result:
[1, 2, 3, 4]
The following use of the delete operator is ok with const as well:
const arr = [10, 20, 30] delete arr[1] // works console.dir(arr)
Result:
[10, <1 empty item>, 30]
Note that delete leaves gaps in the array; for objects it will leave null.
Countless more examples are possible but the key point is: arrays and objects being referenced by a const variable can still be mutated.