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: Assignment to read-only properties is not allowed in strict mode (Edge) TypeError: "x" is read-only (Firefox) TypeError: 0 is read-only (Firefox) TypeError: Cannot assign to read only property 'x' of #<Object> (Chrome) TypeError: Cannot assign to read only property '0' of [object Array] (Chrome)
Error type
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().
'use strict';
var 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';
var 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.
'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:
'use strict';
undefined = function() {}; // TypeError: "undefined" is read-only
Valid cases
'use strict';
var obj = Object.freeze({name: 'Score', points: 157});
obj = {name: obj.name, points: 0}; // replacing it with a new object works
'use strict';
var LUNG_COUNT = 2; // a `var` works, because it's not read-only
LUNG_COUNT = 3; // ok (anatomically unlikely, though)