Master JavaScript objects with this comprehensive cheatsheet! This comprehensive JavaScript object cheatsheet is the perfect resource for anyone who wants to learn everything there is to know about objects in JavaScript. Covering a wide range of topics, from basic object creation to advanced object methods, this cheatsheet has everything you need to start using objects effectively in your code.
JavaScript object key names must adhere to some restrictions to be valid. Key names must either be strings or valid identifier or variable names (i.e. special characters such as -
are not allowed in key names that are not strings).
// Example of invalid key names
const trainSchedule = {
platform num: 10, // Invalid because of the space between words.
40 - 10 + 2: 30, // Expressions cannot be keys.
+compartment: 'C' // The use of a + sign is invalid unless it is enclosed in quotations.
}
Properties of a JavaScript object can be accessed using the dot notation in this manner: object.propertyName
. Nested properties of an object can be accessed by chaining key names in the correct order.
const apple = {
color: 'Green',
price: {
bulk: '$3/kg',
smallQty: '$4/kg'
}
};
console.log(apple.color); // 'Green'
console.log(apple.price.bulk); // '$3/kg'
An object is a built-in data type for storing key-value pairs. Data inside objects are unordered, and the values can be of any type.
When trying to access a JavaScript object property that has not been defined yet, the value of undefined
will be returned by default.
const classElection = {
date: 'January 12'
};
console.log(classElection.place); // undefined
JavaScript objects are mutable, meaning their contents can be changed, even when they are declared as const
. New properties can be added, and existing property values can be changed or deleted.
It is the reference to the object, bound to the variable, that cannot be changed.
const student = {
name: 'Sheldon',
score: 100,
grade: 'A',
}
console.log(student)
// { name: 'Sheldon', score: 100, grade: 'A' }
delete student.score
student.grade = 'F'
console.log(student)
// { name: 'Sheldon', grade: 'F' }
student = {}
// TypeError: Assignment to constant variable.
for...in
loopThe JavaScript for...in
loop can be used to iterate over the keys of an object. In each iteration, one of the properties from the object is assigned to the variable of that loop.
let mobile = {
brand: 'Samsung',
model: 'Galaxy Note 9'
};
for (let key in mobile) {
console.log(`${key}: ${mobile[key]}`);
}
A JavaScript object literal is enclosed with curly braces {}
. Values are mapped to keys in the object with a colon (:
), and the key-value pairs are separated by commas. All the keys are unique, but values are not.
Key-value pairs of an object are also referred to as properties.
const classOf2018 = {
students: 38,
year: 2018
}
Once an object is created in JavaScript, it is possible to remove properties from the object using the delete
operator. The delete
keyword deletes both the value of the property and the property itself from the object. The delete
operator only works on properties, not on variables or functions.
const person = {
firstName: "Matilda",
age: 27,
hobby: "knitting",
goal: "learning JavaScript"
};
delete person.hobby; // or delete person[hobby];
console.log(person);
/*
{
firstName: "Matilda"
age: 27
goal: "learning JavaScript"
}
*/
When JavaScript objects are passed as arguments to functions or methods, they are passed by reference, not by value. This means that the object itself (not a copy) is accessible and mutable (can be changed) inside that function.
const origNum = 8;
const origObj = {color: 'blue'};
const changeItUp = (num, obj) => {
num = 7;
obj.color = 'red';
};
changeItUp(origNum, origObj);
// Will output 8 since integers are passed by value.
console.log(origNum);
// Will output 'red' since objects are passed
// by reference and are therefore mutable.
console.log(origObj.color);
JavaScript objects may have property values that are functions. These are referred to as object methods.
Methods may be defined using anonymous arrow function expressions, or with shorthand method syntax.
Object methods are invoked with the syntax: objectName.methodName(arguments)
.
const engine = {
// method shorthand, with one argument
start(adverb) {
console.log(`The engine starts up ${adverb}...`);
},
// anonymous arrow function expression with no arguments
sputter: () => {
console.log('The engine sputters...');
},
};
engine.start('noisily');
engine.sputter();
/* Console output:
The engine starts up noisily...
The engine sputters...
*/
The JavaScript destructuring assignment is a shorthand syntax that allows object properties to be extracted into specific variable values.
It uses a pair of curly braces ({}
) with property names on the left-hand side of an assignment to extract values from objects. The number of variables can be less than the total properties of an object.
const rubiksCubeFacts = {
possiblePermutations: '43,252,003,274,489,856,000',
invented: '1974',
largestCube: '17x17x17'
};
const {possiblePermutations, invented, largestCube} = rubiksCubeFacts;
console.log(possiblePermutations); // '43,252,003,274,489,856,000'
console.log(invented); // '1974'
console.log(largestCube); // '17x17x17'
The shorthand property name syntax in JavaScript allows creating objects without explicitly specifying the property names (ie. explicitly declaring the value after the key). In this process, an object is created where the property names of that object match variables which already exist in that context. Shorthand property names populate an object with a key matching the identifier and a value matching the identifier’s value.
const activity = 'Surfing';
const beach = { activity };
console.log(beach); // { activity: 'Surfing' }
this
KeywordThe reserved keyword this
refers to a method’s calling object, and it can be used to access properties belonging to that object.
Here, using the this
keyword inside the object function to refer to the cat
object and access its name
property.
const cat = {
name: 'Pipey',
age: 8,
whatName() {
return this.name
}
};
console.log(cat.whatName());
// Output: Pipey
Every JavaScript function or method has a this
context. For a function defined inside of an object, this
will refer to that object itself. For a function defined outside of an object, this
will refer to the global object (window
in a browser, global
in Node.js).
const restaurant = {
numCustomers: 45,
seatCapacity: 100,
availableSeats() {
// this refers to the restaurant object
// and it's used to access its properties
return this.seatCapacity - this.numCustomers;
}
}
JavaScript arrow functions do not have their own this
context, but use the this
of the surrounding lexical context. Thus, they are generally a poor choice for writing object methods.
Consider the example code:
loggerA
is a property that uses arrow notation to define the function. Since data
does not exist in the global context, accessing this.data
returns undefined
.
loggerB
uses method syntax. Since this
refers to the enclosing object, the value of the data
property is accessed as expected, returning "abc"
.
const myObj = {
data: 'abc',
loggerA: () => { console.log(this.data); },
loggerB() { console.log(this.data); },
};
myObj.loggerA(); // undefined
myObj.loggerB(); // 'abc'
JavaScript getter and setter methods are helpful in part because they offer a way to intercept property access and assignment, and allow for additional actions to be performed before these changes go into effect.
const myCat = {
_name: 'Snickers',
get name(){
return this._name
},
set name(newName){
//Verify that newName is a non-empty string before setting as name property
if (typeof newName === 'string' && newName.length > 0){
this._name = newName;
} else {
console.log("ERROR: name must be a non-empty string");
}
}
}
A JavaScript function that returns an object is known as a factory function. Factory functions often accept parameters in order to customize the returned object.
// A factory function that accepts 'name',
// 'age', and 'breed' parameters to return
// a customized dog object.
const dogFactory = (name, age, breed) => {
return {
name: name,
age: age,
breed: breed,
bark() {
console.log('Woof!');
}
};
};
JavaScript object properties are not private or protected. Since JavaScript objects are passed by reference, there is no way to fully prevent incorrect interactions with object properties.
One way to implement more restricted interactions with object properties is to use getter and setter methods.
Typically, the internal value is stored as a property with an identifier that matches the getter and setter method names, but begins with an underscore (_
).
const myCat = {
_name: 'Dottie',
get name() {
return this._name;
},
set name(newName) {
this._name = newName;
}
};
// Reference invokes the getter
console.log(myCat.name);
// Assignment invokes the setter
myCat.name = 'Yankee';
copies properties from one or more source objects to target object
// example
Object.assign({ a: 1, b: 2 }, { c: 3 }, { d: 4 }) // { a: 1, b: 2, c: 3, d: 4 }
// syntax
Object.assign(target, ...sources)
creates new object, using an existing object as the prototype
// example
Object.create({ a: 1 }) // <prototype>: Object { a: 1 }
// syntax
Object.create(proto, [propertiesObject])
defines new or modifies existing properties
// example
Object.defineProperties({ a: 1, b: 2 }, { a: {
value: 3,
writable: true,
}}) // { a: 3, b: 2 }
// syntax
Object.defineProperties(obj, props)
defines new or modifies existing property
// example
Object.defineProperty({ a: 1, b: 2 }, 'a', {
value: 3,
writable: true
}); // { a: 3, b: 2 }
// syntax
Object.defineProperty(obj, prop, descriptor)
returns array of object's [key, value] pairs
// example
Object.entries({ a: 1, b: 2 }) // [ ["a", 1], ["b", 2] ]
// syntax
Object.entries(obj)
freezes an object, which then can no longer be changed
// example
const obj = { a: 1 }
Object.freeze(obj)
obj.prop = 2 // error in strict mode
console.log(obj.prop) // 1
// syntax
Object.freeze(obj)
transforms a list of key-value pairs into an object
// example
Object.fromEntries([['a', 1], ['b', 2]]) // { a: 1, b: 2 }
// syntax
Object.fromEntries(iterable)
returns a property descriptor for an own property
// example
const obj = { a: 1 }
Object.getOwnPropertyDescriptor(obj, 'a') // { value: 1, writable: true, enumerable: true, configurable: true }
// syntax
Object.getOwnPropertyDescriptor(obj, prop)
returns all own property descriptors
// example
const obj = { a: 1 }
Object.getOwnPropertyDescriptors(obj, 'a') // { a: { value: 1, writable: true, enumerable: true, configurable: true } }
// syntax
Object.getOwnPropertyDescriptor(obj, prop)
returns array of all properties
// example
Object.getOwnPropertyNames({ a: 1, b: 2 }) // [ "a", "b" ]
// syntax
Object.getOwnPropertyNames(obj)
array of all symbol properties
// example
const obj = { a: 1 }
const b = Symbol('b')
obj[b] = 'someSymbol' // obj = { a: 1, Symbol(b): "symbol" }
Object.getOwnPropertySymbols(obj) // [ Symbol(b) ]
// syntax
Object.getOwnPropertySymbols(obj)
returns the prototype
// example
const proto = { a: 1 }
const obj = Object.create(proto)
obj.b = 2 // obj = { b: 2 }
Object.getPrototypeOf(obj) // { a: 1 }
// syntax
Object.getPrototypeOf(obj)
determines whether two values are the same value
// example
const objA = { a: 1 }
const objB = { a: 1 }
Object.is(objA, objA) // true
Object.is(objA, objB) // false
Object.is('a', 'a') // true
// syntax
Object.is(value1, value2)
determines wether an object can have new properties added to it
// example
const obj = {}
Object.isExtensible(obj) // true
Object.preventExtensions(obj)
Object.isExtensible(obj) // false
// syntax
Object.isExtensible(obj)
determines if an object is frozen
// example
const obj = {}
Object.isFrozen(obj) // false
Object.freeze(obj)
Object.isFrozen(obj) // true
// syntax
Object.isFrozen(obj)
determines if an object is sealed
// example
const obj = {}
Object.isSealed(obj) // false
Object.seal(obj)
Object.isSealed(obj) // true
// syntax
Object.isSealed(obj)
returns array of object's enumerable property names
// example
Object.keys({ a: 1, b: 2 }) // [ "a", "b" ]
// syntax
Object.keys(obj)
prevents new properties from being added to an object
// example
const obj = { a: 1 }
Object.preventExtensions(obj)
Object.defineProperty(obj, 'b', { value: 2 }) // Error: Can't define property "b": Object is not extensible
// syntax
Object.preventExtensions(obj)
returns boolean indicating whether object has the specified property
// example
const obj = { a: 1 }
obj.hasOwnProperty('a') // true
obj.hasOwnProperty('b') // false
// syntax
obj.hasOwnProperty(prop)
checks if object exists in another object's prototype chain
// example
const proto = { a: 1 }
const obj = Object.create(proto)
proto.isPrototypeOf(obj) // true
// syntax
prototypeObj.isPrototypeOf(object)
checks whether the specified property is enumerable and is the object's own property
// example
const obj = { a: 1 } const arr = ['a']
obj.propertyIsEnumerable('a') // true
arr.propertyIsEnumerable(0) // true
arr.propertyIsEnumerable('length') // false
// syntax
obj.propertyIsEnumerable(prop)
returns a string representing the object
// example
const obj = {}
obj.toString() // "[object Object]"
const arr = ['a', 'b']
arr.toString() // "a,b"
// syntax
obj.toString()
prevents new properties from being added and marks all existing properties as non-configurable
// example
const obj = { a: 1 }
Object.seal(obj)
obj.a = 2 // { a: 2 }
obj.b = 3 // error in strict mode
delete obj.a // error in strict mode
// syntax
Object.seal(obj)
returns array of object's own enumerable property values
// example
Object.values({ a: 1, b: 'a'}) // [ 1, "a" ]
// syntax
Object.values(obj)
const objectName = {};
.
dot notation:objectName.name = "Rabah";
objectName.age = 18;
[]
bracket notation:objectName["name"] = "Rabah";
objectName["age"] = 18;
// another way with variables
const newProp = 'location';
objectName[newProp] = 'FL'
/*
objectName = {
name: "Rabah",
age: 18,
location: "FL"
}
*/
// dot notation
objectName.name; // "Rabah"
// bracket notation
objectName["name"]; // "Rabah"
// extra variation
const propName = "location";
objectName[propName] // "FL"
objectName.age = 25;
objectName["age"] = 25 // 25
for (let property in objectName) {
if (property === ...) {
// Do things here
} else {
// do something otherwise
}
}
const objectKeys = [];
for (let property in objectName) {
objectKeys.push(property);
}
console.log(objectKeys) // ["name", "age", "location"]
// alternatively
const keys = Object.keys(objectName);
console.log(keys) // ["name", "age", "location"]
const objectValues = [];
for (let property in objectName) {
objectValues.push(objectName[property]);
}
console.log(objectValues) // ["Rabah", 25, "FL"]
// alternatively
const values = Object.values(objectName);
console.log(values) // ["Rabah", 25, "FL"]
in
operatorif ("favoriteColor" in objectName) { // if the property favoriteColor exists in objectName
// do something if true
} else {
// do something otherwise
}
// The same way we add properties
objectName.sayBonjour = function(name) {
console.log(`Bonjour ${name}.`);
}
objectName.sayBonjour("Peggy"); // "Bonjour Peggy."
this
keyword // Let's add a new method
objectName.sayMyAge = function() {
this.sayBonjour('Mathieu');
console.log(`You're ${this.age} years old.`);
}
// Now our object looks like this
objectName = {
name: "Rabah",
age: 25,
location: "FL",
sayBonjour: function(name) {
console.log(`Bonjour ${name}.`);
},
sayMyAge: function() {
this.sayBonjour('Mathieu');
// Since the method 'sayBonjour' it's outside of this method scope,
// which is inside the objectName. so to refer to it and use it by calling it, we have to use the 'this' keyword.
// Using the 'this.sayBonjour()' keyword is the same thing as saying:
// objectName.sayBonjour()
console.log(`You're ${this.age} years old.`);
// The same thing happening here. We're saying '`I'm ${objectName.age} years old.'
}
}
//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
objectName.sayMyAge(); // "Bonjour Mathieu."
// "I'm 25 years old."
#javascript #js