Message
TypeError: invalid assignment to const "x" (Firefox) TypeError: Assignment to constant variable. (Chrome) TypeError: Redeclaration of const 'x' (IE/Edge)
Error type
What went wrong?
A constant is a value that cannot be altered by the program during normal execution. It cannot change through re-assignment, and it can't be redeclared. In JavaScript, constants are declared using the const keyword.
Examples
Invalid redeclaration
Assigning a value to the same constant name in the same block-scope will throw.
const COLUMNS = 80; // ... COLUMNS = 120; // TypeError: invalid assignment to const `COLUMNS'
Fixing the error
There are multiple options to fix this error. Check what was intended to be achieved with the constant in question.
Rename
If you meant to declare another constant, pick another name and re-name. This constant name is already taken in this scope.
const COLUMNS = 80; const WIDE_COLUMNS = 120;
const, let or var?
Do not use const if you weren't meaning to declare a constant. Maybe you meant to declare a block-scoped variable with let or global variable with var.
let columns = 80; // ... let columns = 120;
Scoping
Check if you are in the correct scope. Should this constant appear in this scope or was is meant to appear in a function, for example?
const COLUMNS = 80;
function setupBigScreenEnvironment() {
const COLUMNS = 120;
}
const and immutability
The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned. For instance, in case the content is an object, this means the object itself can still be altered. This means that you can't mutate the value stored in a variable:
const obj = {foo: 'bar'};
obj = {foo: 'baz'}; // TypeError: invalid assignment to const `obj'
But you can mutate the properties in a variable:
obj.foo = 'baz';
obj; // Object { foo: "baz" }