delete

In JavaScript, the delete keyword is a unary operator that removes a property from an object. It returns true when the property is gone afterward, including when it never existed, and false when the property is non-configurable and can’t be removed.

The operator only affects the object’s own properties. After deleting an own property, a read of the same name finds the inherited value from the prototype, if one exists. Deleting an array element leaves a hole, so the array’s .length doesn’t change. The delete operator doesn’t free memory directly. The garbage collector reclaims a value once nothing references it.

In strict mode, which includes modules and class bodies, two cases are errors. Deleting a plain variable name, such as delete count, is a SyntaxError, and deleting a non-configurable property throws a TypeError. In sloppy mode, both fail quietly. Variables declared with let, const, or var can’t be deleted in either mode.

delete <object>.<property>
delete <object>[<expression>]

> const user = { name: "Ada", password: "hunter2" };
undefined
> delete user.password
true
> user
{ name: 'Ada' }

> const colors = ["red", "green", "blue"];
undefined
> delete colors[1]
true
> colors
[ 'red', <1 empty item>, 'blue' ]
> colors.length
3

> delete Math.PI
false

delete Math.PI;

$ node constants.mjs
TypeError: Cannot delete property 'PI' of #<Object>