JS: delete (operator) ❌

By Xah Lee. Date: 2017-02-15. Last updated: 2026-01-25.

delete obj[key]

Deletes the property key from obj.

Return true when:
Property exist and is deleted.
Property does not exist.
Argument is not an object.

else return false.

🛑 WARNING: using delete operator on array results a Sparse Array.

🟢 TIP: The delete operator never goes up the Prototype Chain.

const jj = { kk: 1 };

// delete the property kk
delete jj.kk;

console.assert(Object.hasOwn(jj, "kk") === false);

delete obj.key

similar to delete obj[key]. 〔see Property Dot Notation vs Bracket Notation〕

Edge cases

On non-object

// warning. Using delete on non-object return true
console.assert((delete 3) === true);

On non-exist property

// warning. Using delete on non-exist property return true
const jj = {};
console.assert((delete jj.k) === true);

On frozen object

JS: Object.freeze

// example of failure due to frozen object
const jj = { pp: 1 };
Object.freeze(jj);

try {
 delete jj.pp;
} catch (xerr) {
 console.log("delete property pp failed.\n", xerr);
}

On array, result sparse array

JS: Sparse Array

// using delete operator results a sparse array
const xx = [0, 1, 2, 3];
delete xx[0];
console.assert(Object.hasOwn(xx, "0") === false);

🟢 TIP: Never Use the Delete Operator

Do not use the delete operator to delete array element. That results a Sparse Array . Use Array.prototype.splice to delete elements in array. Best is actually set items to null.
Delete operator should only be used to delete object property. But Reflect.deleteProperty is better, because it has more sensible return value.
Avoid deleting or adding properties to object frequently, as in a loop. Because, due to implementation, adding/deleting properties is inefficient. Instead, change the value to null.