Message
TypeError: can't delete non-configurable array element (Firefox) TypeError: Cannot delete property '2' of [object Array] (Chrome)
Error type
What went wrong?
It was attempted to shorten the length of an array, but one of the array's elements is non-configurable. When shortening an array, the elements beyond the new array length will be deleted, which failed in this situation.
The configurable attribute controls whether the property can be deleted from the object and whether its attributes (other than writable) can be changed.
Usually, properties in an object created by an array initializer are configurable. However, for example, when using Object.defineProperty(), the property isn't configurable by default.
Examples
Non-configurable properties created by Object.defineProperty
The Object.defineProperty() creates non-configurable properties by default if you haven't specified them as configurable.
var arr = [];
Object.defineProperty(arr, 0, {value: 0});
Object.defineProperty(arr, 1, {value: "1"});
arr.length = 1;
// TypeError: can't delete non-configurable array element
You will need to set the elements as configurable, if you intend to shorten the array.
var arr = [];
Object.defineProperty(arr, 0, {value: 0, configurable: true});
Object.defineProperty(arr, 1, {value: "1", configurable: true});
arr.length = 1;
Seal-ed Arrays
The Object.seal() function marks all existing elements as non-configurable.
var arr = [1,2,3]; Object.seal(arr); arr.length = 1; // TypeError: can't delete non-configurable array element
You either need to remove the Object.seal() call, or make a copy of it. In case of a copy, shortening the copy of the array does not modify the original array length.
var arr = [1,2,3]; Object.seal(arr); // Copy the initial array to shorten the copy var copy = Array.from(arr); copy.length = 1; // arr.length == 3