Which of the following is the operator used to determine whether an object?

prop in object #prop in object

class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 9

A string or symbol representing a property name (non-symbols will be ). Can also be a private property identifier.

Object to check if it (or its prototype chain) contains the property with specified name (class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 9).

class C { #x; static isC(obj) { return #x in obj; } } 2

Thrown if class C { #x; static isC(obj) { return #x in obj; } } 0 is not an object (i.e. a primitive).

The class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 7 operator tests if a string or symbol property is present in an object or its prototype chain. If you want to check for only non-inherited properties, use class C { #x; static isC(obj) { return #x in obj; } } 5 instead.

A property may be present in an object but have value class C { #x; static isC(obj) { return #x in obj; } } 6. Therefore, class C { #x; static isC(obj) { return #x in obj; } } 7 is not the same as class C { #x; static isC(obj) { return #x in obj; } } 8. To make class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 7 return class C { foo() { #x in this; } } new C().foo(); // SyntaxError: Private field '#x' must be declared in an enclosing class 0 after a property is added, use the class C { foo() { #x in this; } } new C().foo(); // SyntaxError: Private field '#x' must be declared in an enclosing class 1 operator instead of setting that property's value to class C { #x; static isC(obj) { return #x in obj; } } 6.

You can also use the class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 7 operator to check whether a particular private class field or method has been defined in an object. The operator returns class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 8 if the property is defined, and class C { foo() { #x in this; } } new C().foo(); // SyntaxError: Private field '#x' must be declared in an enclosing class 0 otherwise. This is known as a branded check, because it returns class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 8 if and only if the object was created with that class constructor, after which you can safely access other private properties as well.

This is a special syntax — the left-hand side of the class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 7 operator is a property identifier instead of an expression, but unquoted (because otherwise it's a string property, not a private property).

Because accessing private properties on objects unrelated to the current class throws a class C { #x; static isC(obj) { return #x in obj; } } 2 instead of returning class C { #x; static isC(obj) { return #x in obj; } } 6, this syntax allows you to shorten:

class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } }

To:

class C { #x; static isC(obj) { return #x in obj; } }

It also generally avoids the need for dealing with error handling just to access a private property that may be nonexistent.

However, the class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 7 operator still requires the private property to be declared beforehand in the enclosing class — otherwise, it would throw a // Arrays const trees = ["redwood", "bay", "cedar", "oak", "maple"]; 0 in trees; // returns true 3 in trees; // returns true 6 in trees; // returns false "bay" in trees; // returns false (you must specify the index number, not the value at that index) "length" in trees; // returns true (length is an Array property) Symbol.iterator in trees; // returns true // Predefined objects "PI" in Math; // returns true // Custom objects const mycar = { make: "Honda", model: "Accord", year: 1998 }; "make" in mycar; // returns true "model" in mycar; // returns true 1 ("Private field '#x' must be declared in an enclosing class"), the same one as when you try to access an undeclared private property.

class C { foo() { #x in this; } } new C().foo(); // SyntaxError: Private field '#x' must be declared in an enclosing class

The following examples show some uses of the class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 7 operator.

// Arrays const trees = ["redwood", "bay", "cedar", "oak", "maple"]; 0 in trees; // returns true 3 in trees; // returns true 6 in trees; // returns false "bay" in trees; // returns false (you must specify the index number, not the value at that index) "length" in trees; // returns true (length is an Array property) Symbol.iterator in trees; // returns true // Predefined objects "PI" in Math; // returns true // Custom objects const mycar = { make: "Honda", model: "Accord", year: 1998 }; "make" in mycar; // returns true "model" in mycar; // returns true

You must specify an object on the right side of the class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 7 operator. For example, you can specify a string created with the // Arrays const trees = ["redwood", "bay", "cedar", "oak", "maple"]; 0 in trees; // returns true 3 in trees; // returns true 6 in trees; // returns false "bay" in trees; // returns false (you must specify the index number, not the value at that index) "length" in trees; // returns true (length is an Array property) Symbol.iterator in trees; // returns true // Predefined objects "PI" in Math; // returns true // Custom objects const mycar = { make: "Honda", model: "Accord", year: 1998 }; "make" in mycar; // returns true "model" in mycar; // returns true 4 constructor, but you cannot specify a string literal.

const color1 = new String("green"); "length" in color1; // returns true const color2 = "coral"; // generates an error (color2 is not a String object) "length" in color2;

If you delete a property with the class C { foo() { #x in this; } } new C().foo(); // SyntaxError: Private field '#x' must be declared in an enclosing class 1 operator, the class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 7 operator returns class C { foo() { #x in this; } } new C().foo(); // SyntaxError: Private field '#x' must be declared in an enclosing class 0 for that property.

const mycar = { make: "Honda", model: "Accord", year: 1998 }; delete mycar.make; "make" in mycar; // returns false const trees = ["redwood", "bay", "cedar", "oak", "maple"]; delete trees[3]; 3 in trees; // returns false

If you set a property to class C { #x; static isC(obj) { return #x in obj; } } 6 but do not delete it, the class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 7 operator returns true for that property.

const mycar = { make: "Honda", model: "Accord", year: 1998 }; mycar.make = undefined; "make" in mycar; // returns true

const trees = ["redwood", "bay", "cedar", "oak", "maple"]; trees[3] = undefined; 3 in trees; // returns true

The class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 7 operator will return class C { foo() { #x in this; } } new C().foo(); // SyntaxError: Private field '#x' must be declared in an enclosing class 0 for , even if accessing it directly returns class C { #x; static isC(obj) { return #x in obj; } } 6.

const empties = new Array(3); empties[2]; // returns undefined 2 in empties; // returns false

To avoid this, make sure a new array is always filled with non-empty values or not write to indexes past the end of array.

class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 0

The class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 7 operator returns class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 8 for properties in the prototype chain. This may be undesirable if you are using objects to store arbitrary key-value pairs.

class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 1

You can use class C { #x; static isC(obj) { return #x in obj; } } 5 to check if the object has the key.

class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 2

Alternatively, you should consider using a or a const color1 = new String("green"); "length" in color1; // returns true const color2 = "coral"; // generates an error (color2 is not a String object) "length" in color2; 6 for storing const color1 = new String("green"); "length" in color1; // returns true const color2 = "coral"; // generates an error (color2 is not a String object) "length" in color2; 7, to avoid other bugs.

class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 3

The code fragment below demonstrates a static function that tells if an object was created with the const color1 = new String("green"); "length" in color1; // returns true const color2 = "coral"; // generates an error (color2 is not a String object) "length" in color2; 8 constructor and therefore can perform other methods safely.

class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 4

It helps to prevent the following case:

class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 5

Without the class C { #x; static isC(obj) { try { obj.#x; return true; } catch { return false; } } } 7 operator, you would have to use a const mycar = { make: "Honda", model: "Accord", year: 1998 }; delete mycar.make; "make" in mycar; // returns false const trees = ["redwood", "bay", "cedar", "oak", "maple"]; delete trees[3]; 3 in trees; // returns false 0 block to check if the object has the private property.

You can also implement this as a const mycar = { make: "Honda", model: "Accord", year: 1998 }; delete mycar.make; "make" in mycar; // returns false const trees = ["redwood", "bay", "cedar", "oak", "maple"]; delete trees[3]; 3 in trees; // returns false 1 method of the class, so that you can use the const mycar = { make: "Honda", model: "Accord", year: 1998 }; delete mycar.make; "make" in mycar; // returns false const trees = ["redwood", "bay", "cedar", "oak", "maple"]; delete trees[3]; 3 in trees; // returns false 2 operator to perform the same check (which, by default, only checks for the existence of const mycar = { make: "Honda", model: "Accord", year: 1998 }; delete mycar.make; "make" in mycar; // returns false const trees = ["redwood", "bay", "cedar", "oak", "maple"]; delete trees[3]; 3 in trees; // returns false 3 in the object's prototype chain).

Which of the following operators can you use to test if an object has a specific type?

What object oriented programming characteristic allows you to create a class that is a specialized version of another class?

What does instance of mean in Java?

What is the return type of the instance of operator?