Javascript Cheatsheet




Data Types

Javascript is a dynamically typed language and hence though there are data types, variables are not bound to them.

Data TypeDescription
numberRepresents numbers like integers, floating-values etc
stringRepresents one or more characters
bigintRepresents integers of arbitrary length
nullRepresents unknown values
undefinedRepresents undefined values
objectRepresents complex data structures
booleanRepresents either true or false

Variables

KeywordDescriptionScope
varVar is used to declare variables(old way of declaring variables)Function or global scope
letlet is also used to declare variables(new way)Global or block Scope
constconst is used to declare const values. Once the value is assigned it can not be modifiedGlobal or block Scope
let variable-name; // Just declaration
let variable-name = value; // declaring variable and assigning it with some value
let var1 = value1, var2 = value2, var3 = value3; // multiple variables declaration with their assigned values

Basics

On page script

<script type="text/javascript">  ...
</script>

To include external javascript file

<script src="filename.js"></script>

Comments

  • Single line comments - //
  • Multi line comments - /* */

Output

console.log("hello");       // to display message to the browser console
document.write(x);    // to write to HTML
prompt("Your name?","friend");    // input dialog. Second argument is the initial value
alert(x); // writes in an alert box

Strict mode

"use strict";  // throw errors for some of javascript silent errors and tells browser to be more strict.

Conditional Statements

If

let age = 20; 

if (age > 18) {
  console.log('Adult');
}

If-else

let age=7;

if(age<0) {
console.log('invalid age');
} else if (age<5 && age >0) {
console.log('Infant');
} else if (age>=5 && age<=18) {
  console.log('child');
} else if (age>18) {
  console.log("Adult");
}

Switch

// what is hello in different languages
let language = "italian";
switch(language){
  case "french" :
    console.log('BONJOUR');
    break;
  
  case "spanish":
    console.log('Hola');
    break;
  
  case "hindi" :
    console.log("Namaste");
    break;
  
  default:
    console.log('Hello');
}

Loops

for

console.log('simple for loop');
for(let i=1;i<=10;i++) {
  console.log(i);
}

for..in

let info = {
  name: "foo",
  id: 123
}

for (let x in info) {
  console.log(x); // prints keys of info which are name and id
}

for..of

let mobiles = [ "iPhone", "Samsung", "OnePlus", "Pixel"];
for(let mbl of mobiles) {  
    console.log(mbl);
} 

while

let i=1;
while(i<=10) {
  console.log(i);
  i++;
}

do-while

let i=1;
do {
  console.log(i);
  i++;
} while(i<=10);

Operators

TypeOperators
Arithmetic Operators+ - * / % ++ --
Comparision Operators== === != !== > >= < <=
Bitwise Operators& ^ | ^ ~ << >> >>>
Logical Operators&& || !
Assignment Operators= += -= *= /= %=
Special Operators? :, ,, new, typeof, void, yield, delete, in, instanceof

Arrays

let arrayName = [value1, value2,..etc];
// or
let arrayName = new Array("value1","value2",..etc);

Array Methods

Method nameSyntax
forEach()arrayname.forEach(function(err, doc){ //code });
map()arrayName.map(function(err, doc)){ //code });
filter()arrayName.filter(function(err, doc)) { //code });
reduce()arrayName.reduce(function(err, doc)) { //code });
find()arrayName.find(function(err, doc)) { //code });
indexOf()arrayName.indexOf(element);
from()Array.from(arrayLike[, mapFn, thisArg]);
every()Array.every(callback(element[, index, array]), thisArg)
some()Array.some(callback(element[, index, array]), thisArg)
includes()arrayName.includes(value-to-be-checked, starting-search-index)

Functions

// declaring a function
function function-name(parameters){ // here parameters are optional
    //code
}
function-name(parameters); // calling a function

Arrow function

(argument-list) => expression

Example

let sum= (a,b,c) => {
    return a+b+c;
}
console.log(sum(10,20,30));

Methods

String Methods

Method NameUsage
search()let sindex=str.search("sub-string");
slice(start,end)let sub-str=str.slice(starting position,ending position);
substring(start,endlet sub-str=str.substring(starting position,ending position);
substr(start,length)let sub-str=str.substr(starting position,length);
trim()let str1= str.trim();
charAt()let c=str.charAt(position);
charCodeAt()let c=str.charCodAt(position);
split()let array=str.split("");
lengthlet vln = str.length()
indexOf()let index=str.indexOf("sub-string");
lastIndexOf()let index=str.lastIndexOf("sub-string");
toUpperCase()let str1=str.toUpperCase();
toLowerCase()let str1=str.toLowerCase();
replace()let x=str.replace("string to replace","replacement string")
concat()let str3=str1.concat(" ",str2);

Number related Methods

Method NameUsage
Number()Number(x);
parseInt()parseInt(x);
parseFloat()parseFloat(x);
toString()let x = num.toString();
toFixed()let x=num.toFixed(no of decimals)
toExponential()let x=num.toExponential();
toPrecision()let x=num.toPrecision(length)
valueOf()let x= num.valueOf();

Date Methods

considering date = new Date() for the below methods.

MethodUsagecomments
getDay()date.getDay()returns day as number 0 to 6
Date.now()let now = Date.now()returns date and time
getFullYear()date.getFullYear();returns yyyy i.e., 2020
setFullYear()let year = date.setFullYear(2020);sets year as 2020
getMonth()date.getMonth();returns month as a number (0-11)
setMonth()let month = date.setMonth(10);sets month as 10 means november
getDate()date.getDate();retuns date as number 1 to 31
setDate()let day = date.setDate(20);sets date as 20
getHours()date.getHours();to get the hour (0-23)
setHours()let hrs = date.setHours(20);to set the hour (0-23)
getMinutes()date.getMinutes();to get the minute (0-59)
setMinutes()let min = date.setMinutes(40);to set the minutes (0-59)
getSeconds()date.getSeconds();to get the second (0-59)
setSeconds()let sec = date.setSeconds(30);to set the seconds (0-59)
getMilliseconds()date.getMilliseconds();to get the millisecond (0-999)
setMilliseconds()let milli = date.setMilliseconds(500);to set the milliseconds (0-999)
setTime()let dateTime = date.setTime(1582268856705);to set the time (milliseconds since January 1, 1970)
getTime()date.getTime()to get the time (milliseconds since January 1, 1970)

Math functions

FunctionComments
Math.PI;returns pi value 3.141592653589793
Math.round(10.4);returns 10
Math.round(10.5);returns 5
Math.pow(2,3);returns 8 which is 2 to the power of 3
Math.sqrt(100);returns 10
Math.abs(-5.3);returns 5.3
Math.ceil(22.12);returns 23 by rounding up
Math.floor(22.92);returns 22 by rounding down
Math.min(2, 5, -7, 3);returns the lowest value which is -7
Math.max(2, 5, -7, 3);returns the highest value which is 5
Math.log(1);returns log value as 0
Math.random();returns a random number between 0 and 1
Math.sin(0);returns 0
Math.cos(Math.PI);to use tan,atan,asin,acos

Promises

let promise = new Promise(function(resolve, reject){
    //code
}); 

Async-Await

Syntax

async function functioname(parameters){
	//code
}

Example

async getTodos(userObj){
	const res = await fetch([url]);
	const data = await res.json()    
	return data;
}

let data = await getTodos({fn: "foo"});

Error Handling

try {
    //code
} catch(err) {
    //code
}

throw error

throw "Error message";    // throw error text to user

Regular expressions

Syntax

/pattern/modifiers;
ModifiersDescription
gPerforms a global match and finds all
iPerforms case-insensitive matching
mPerforms multiline matching


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