TypeError: "x" is read-only
The JavaScript strict mode-only exception "is read-only" occurs when a global variable or object property that was assigned to is a read-only property.
Message
TypeError: Cannot assign to read only property 'x' of #<Object> (V8-based) TypeError: "x" is read-only (Firefox) TypeError: Attempted to assign to readonly property. (Safari)
Error type
TypeError
in strict mode only.
What went wrong?
The global variable or object property that was assigned to is a read-only property. (Technically, it is a non-writable data property.)
This error happens only in strict mode code. In non-strict code, the assignment is silently ignored.
Examples
Invalid cases
Read-only properties are not super common, but they can be created using
Object.defineProperty()
or Object.freeze()
.
js
"use strict";
const obj = Object.freeze({ name: "Elsa", score: 157 });
obj.score = 0; // TypeError
("use strict");
Object.defineProperty(this, "LUNG_COUNT", { value: 2, writable: false });
LUNG_COUNT = 3; // TypeError
("use strict");
const frozenArray = Object.freeze([0, 1, 2]);
frozenArray[0]++; // TypeError
There are also a few read-only properties built into JavaScript. Maybe you tried to redefine a mathematical constant.
js
"use strict";
Math.PI = 4; // TypeError
Sorry, you can't do that.
The global variable undefined
is also read-only, so you can't silence the
infamous "undefined is not a function" error by doing this:
js
"use strict";
undefined = function () {}; // TypeError: "undefined" is read-only
Valid cases
js
"use strict";
let obj = Object.freeze({ name: "Score", points: 157 });
obj = { name: obj.name, points: 0 }; // replacing it with a new object works