JavaScript, often abbreviated JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for webpage behavior, often incorporating third-party libraries.
let const break continue debugger
if else switch case default
while do for in of
function return var try catch
throw finally class extends super
import export default true false
null undefined typeof instanceof
+ - * ** / % ++
-- = += -= *= /=
%= == === != !== < >
<= >= ? : && || !
& | ^ ~ << >>
>>>
&= |= ^= <<= >>= >>>=
( ) [ ] { }
, ; : . ` =
' " / \ @
// Operator Meaning Usage
+ Addition or unary plus x + y
- Subtraction or unary minus x - y
* Multiplication x * y
/ Division (result is always a float) x / y
% Modulus x % y (remainder of x/y)
** Exponent x**y (x to the power y)
// Operator Description
** Exponentiation
++, -- Increment, Decrement
+, -, ~ Unary Plus, Unary Minus, Bitwise NOT
*, /, % Multiplication, Division, Modulo
+, - Addition, Subtraction
<<, >>, >>> Bitwise Shifts
& Bitwise AND
^ Bitwise XOR
| Bitwise OR
<, <=, >, >= Relational operators
==, ===, !=, !== Equality operators
&& Logical AND
|| Logical OR
?: Conditional (Ternary) operator
=, +=, -=, *=, /=, %=, <<=, >>=, >>>= Assignment operators
, Comma operator
// Operator Meaning Usage
> Greater than x > y
< Less than x < y
== Equal to x == y
=== Strict Equal to x === y
!= Not equal to x != y
!== Strict Not equal to x !== y
>= Greater than or equal to x >= y
<= Less than or equal to x <= y
// Operator Meaning Usage
&& Logical AND x && y
|| Logical OR x || y
! Logical NOT !x
A !A
true false
false true
A B A && B
true true true
true false false
false true false
false false false
A B A || B
true true true
true false true
false true true
false false falseOperator Meaning Usage
in Check to see if value is in the sequence 5 in [2,5,3,7]
instanceof Check to see if an object is an instance of a class obj instanceof MyClass
OPERATOR DESCRIPTION
() Parenthesis (grouping)
f(args...), x[i:i], x[i], x.attr Function call, slicing, subscript, dot
++, -- Increment, Decrement
+, -, ~ Unary Plus, Unary Minus, Bitwise NOT
*, /, % Multiplication, Division, Modulo
+, - Addition, Subtraction
<<, >>, >>> Bitwise Shifts
& Bitwise AND
^ Bitwise XOR
| Bitwise OR
<, <=, >, >=, instanceof Relational operators
==, ===, !=, !== Equality operators
&& Logical AND
|| Logical OR
?: Conditional (Ternary) operator
=, +=, -=, *=, /=, %=, <<=, >>=, >>>= Assignment operators
, Comma operator
Functions are one of the fundamental building blocks in JavaScript. A function in JavaScript is similar to a procedure—a set of statements that performs a task or calculates a value, but for a procedure to qualify as a function, it should take some input and return an output where there is some obvious relationship between the input and the output
Object.assign(target, ...sources): Copies the values of all enumerable own properties from one or more source objects to a target object.
Object.create(proto, [propertiesObject]): Creates a new object with the specified prototype object and properties.
Object.entries(obj): Returns an array of key-value pairs for an object's enumerable string-keyed properties.
Object.keys(obj): Returns an array of a given object's own enumerable property names.
Object.values(obj): Returns an array of a given object's own enumerable property values.
Object.freeze(obj): Freezes an object, preventing new properties from being added and existing properties from being removed or changed.
Object.seal(obj): Seals an object, preventing new properties from being added and marking all existing properties as non-configurable.
Object.is(obj1, obj2): Compares two values for equality. Returns true if they are the same value.
Object.hasOwnProperty(prop): Returns a boolean indicating whether an object has the specified property as its own property.
Object.getOwnPropertyNames(obj): Returns an array of all properties (enumerable or not) found directly upon a given object.
Object.getPrototypeOf(obj): Returns the prototype (i.e., the value of the internal [[Prototype]] property) of the specified object.
Object.setPrototypeOf(obj, prototype): Sets the prototype (i.e., the internal [[Prototype]] property) of a specified object to another object or null.
Object.isPrototypeOf(obj): Returns a boolean indicating whether the object this method is called upon is in the prototype chain of the specified object.
Object.toString(): Returns a string representing the object.
Object.valueOf(): Returns the primitive value of the specified object.
Object.defineProperty(obj, prop, descriptor): Adds the named property or modifies the attributes of the named property of an object.
Object.defineProperties(obj, props): Adds the named properties to an object, or modifies the attributes of the existing properties.
Object.getOwnPropertyDescriptor(obj, prop): Returns a property descriptor for a named property on an object.
Object.getOwnPropertySymbols(obj): Returns an array of all symbol properties found directly upon a given object.
Object.getOwnPropertyDescriptors(obj): Returns an object containing all own property descriptors for an object.
Object.isExtensible(obj): Determines if an object is extensible (whether it can have new properties added to it).
Object.preventExtensions(obj): Prevents new properties from ever being added to an object.
Math.abs() Math.min() Math.max() Math.ceil() Math.floor()
Math.random() Math.round() Math.sqrt() Math.pow() Math.log()
Math.sin() Math.cos() Math.tan() Math.asin() Math.acos()
Math.atan() Math.exp() Math.PI Math.E
str.length: Returns the length of the string.
str.charAt(index): Returns the character at the specified index in the string.
str.concat(str1, str2, ..., strN): Concatenates two or more strings.
str.indexOf(searchValue, fromIndex): Returns the index of the first occurrence of a specified substring.
str.lastIndexOf(searchValue, fromIndex): Returns the index of the last occurrence of a specified substring.
str.slice(start, end): Extracts a portion of the string, starting at the specified index and ending at another specified index.
str.substring(start, end): Returns a new string containing characters from start to end (excluding end).
str.substr(start, length): Extracts a portion of the string, starting at the specified index and extending for a given number of characters.
str.replace(searchValue, replaceValue): Replaces a specified value with another value in a string.
str.toUpperCase(): Converts a string to uppercase letters.
str.toLowerCase(): Converts a string to lowercase letters.
str.trim(): Removes whitespace from both ends of a string.
str.startsWith(searchString, position): Checks if a string starts with a specified substring.
str.endsWith(searchString, position): Checks if a string ends with a specified substring.
str.includes(searchString, position): Checks if a string contains a specified substring.
str.split(separator, limit): Splits a string into an array of substrings based on a specified separator.
str.match(regexp): Retrieves the result of matching a string against a regular expression.
str.search(regexp): Searches a string for a specified value and returns the position of the match.
str.localeCompare(compareString[, locales[, options]]): Compares two strings in the current locale.
str.repeat(count): Returns a new string with a specified number of copies of the string.
str.padStart(targetLength, padString): Pads the current string with another string (multiple times, if needed) until the resulting string reaches the given length.
str.padEnd(targetLength, padString): Pads the current string with another string (multiple times, if needed) until the resulting string reaches the given length.
arr.length // Returns the length of the array
arr.push(6) // Adds a new element to the end of the array
arr.pop() // Removes the last element from the array
arr.unshift(0) // Adds a new element to the beginning of the array
arr.shift() // Removes the first element from the array
arr.join(", ") // Joins all elements of an array into a string
arr.indexOf(3) // Returns the index of the first occurrence of the specified element
arr.lastIndexOf(3) // Returns the index of the last occurrence of the specified element
arr.slice(start, end) // Extracts a portion of the array, starting at the specified index and ending at another specified index
arr.splice(start, deleteCount, item1, item2, ...) // Changes the contents of an array by removing or replacing existing elements and/or adding new elements
arr.sort() // Sorts the elements of an array
arr.reverse() // Reverses the order of the elements in an array
arr.concat(array1, array2 ..., arrayN) // Returns a new array consisting of the elements of the original array followed by one or more arrays or values.
arr.forEach(callback(currentValue, index, array)) //Executes a provided function once for each array element.
arr.map(callback(currentValue, index, array)) //Creates a new array with the results of calling a provided function on every element in the array.
arr.filter(callback(element, index, array)) //Creates a new array with all elements that pass the test implemented by the provided function.
arr.reduce(callback(accumulator, currentValue, index, array), initialValue //Applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value.
arr.reduceRight(callback(accumulator, currentValue, index, array), initialValue) //Applies a function against an accumulator and each element in the array (from right to left) to reduce it to a single value.
arr.every(callback(element, index, array)) //Tests whether all elements in the array pass the test implemented by the provided function.
arr.some(callback(element, index, array)) // Tests whether at least one element in the array passes the test implemented by the provided function.
arr.includes(searchElement, fromIndex)// Determines whether an array includes a certain element, returning true or false as appropriate.
let date = new Date();
date.getFullYear() // Returns the year of a date as a four-digit number
date.getMonth() // Returns the month of a date as a number (0-11)
date.getDate() // Returns the day of the month of a date (1-31)
date.getHours() // Returns the hour of a date (0-23)
date.getMinutes() // Returns the minutes of a date (0-59)
date.getSeconds() // Returns the seconds of a date (0-59)
date.getMilliseconds()// Returns the milliseconds of a date (0-999)
date.getDay() // Returns the day of the week of a date (0-6, where Sunday is 0)
date.getTime() // Returns the numeric value of the specified date as the number of milliseconds since January 1, 1970
date.toString() // Converts a date to a string
const regex = /pattern/;
const isMatch = regex.test("input string");
console.log(isMatch); // Output: true or false
const regex = /pattern/;
const matchInfo = regex.exec("input string");
console.log(matchInfo); // Output: Array or null
const regex = /pattern/g; // Note the 'g' flag for global matching
const matches = "input string".match(regex);
console.log(matches); // Output: Array or null
const regex = /pattern/;
const index = "input string".search(regex);
console.log(index); // Output: Index or -1
const regex = /pattern/;
const replacedString = "input string".replace(regex, "replacement");
console.log(replacedString); // Output: Modified string
const caseInsensitivePattern = /pattern/i;
const globalPattern = /pattern/g;
const multilinePattern = /^pattern/m;
const fetchData = () => {
return new Promise((resolve, reject) => {
setTimeout(() => {
const data = 'Async data';
// Simulating success
resolve(data);
// Simulating an error
// reject(new Error('Failed to fetch data'));
}, 1000);
});
};
fetchData()
.then((result) => {
console.log('Success:', result);
})
.catch((error) => {
console.error('Error:', error.message);
});
const myAsyncFunction = async () => {
try {
const result = await somePromiseFunction();
// Code here runs after the promise is resolved
console.log(result);
} catch (error) {
// Handle errors
console.error(error);
}
};
const fetchData = () => {
return new Promise((resolve, reject) => {
setTimeout(() => {
const data = 'Async data';
// Simulating success
resolve(data);
// Simulating an error
// reject(new Error('Failed to fetch data'));
}, 1000);
});
};
const fetchDataAsync = async () => {
try {
const result = await fetchData();
console.log('Success:', result);
} catch (error) {
console.error('Error:', error.message);
}
};
fetchDataAsync();