TypeError: can't delete non-configurable array element

The JavaScript exception "can't delete non-configurable array element" occurs when it was attempted to shorten the length of an array, but one of the array's elements is non-configurable.

Message

TypeError: Cannot delete property '1' of [object Array] (V8-based)
TypeError: can't delete non-configurable array element (Firefox)
TypeError: Unable to delete property. (Safari)

Error type

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 Object.defineProperty() creates non-configurable properties by default if you haven't specified them as configurable.

js
"use strict";
const 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.

js
"use strict";
const arr = [];
Object.defineProperty(arr, 0, { value: 0, configurable: true });
Object.defineProperty(arr, 1, { value: "1", configurable: true });

arr.length = 1;

Sealed Arrays

The Object.seal() function marks all existing elements as non-configurable.

js
"use strict";
const 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.

js
"use strict";
const arr = [1, 2, 3];
Object.seal(arr);

/ Copy the initial array to shorten the copy
const copy = Array.from(arr);
copy.length = 1;
/ arr.length === 3

See also