JS CheatSheet
Basics➤
On page script
<script type="text/javascript"> ...
</script>
Include external JS file
<script src="filename.js"></script>
Delay - 1 second timeout
setTimeout(function () {
}, 1000);
Functions
function addNumbers(a, b) {
return a + b; ;
}
x = addNumbers(1, 2);
Edit DOM element
document.getElementById("elementID").innerHTML = "Hello World!";
Output
console.log(a); // write to the browser console
document.write(a); // write to the HTML
alert(a); // output in an alert box
confirm("Really?"); // yes/no dialog, returns true/false depending on user click
prompt("Your age?","0"); // input dialog. Second argument is the initial value
Comments
/* Multi line
comment */
// One line
Variablesx
var a; // variable
var b = "init"; // string
var c = "Hi" + " " + "Joe"; // = "Hi Joe"
var d = 1 + 2 + "3"; // = "33"
var e = [2,3,5,8]; // array
var f = false; // boolean
var g = /()/; // RegEx
var h = function(){}; // function object
const PI = 3.14; // constant
var a = 1, b = 2, c = a + b; // one line
let z = 'zzz'; // block scope local variable
Strict mode
"use strict"; // Use strict mode to write secure code
x = 1; // Throws an error because variable is not declared
Values
false, true // boolean
18, 3.14, 0b10011, 0xF6, NaN // number
"flower", 'John' // string
undefined, null , Infinity // special
Operators
a = b + c - d; // addition, substraction
a = b * (c / d); // multiplication, division
x = 100 % 48; // modulo. 100 / 48 remainder = 4
a++; b--; // postfix increment and decrement
Bitwise operators
& | AND | 5 & 1 (0101 & 0001) | 1 (1) |
| | OR | 5 | 1 (0101 | 0001) | 5 (101) |
~ | NOT | ~ 5 (~0101) | 10 (1010) |
^ | XOR | 5 ^ 1 (0101 ^ 0001) | 4 (100) |
<< | left shift | 5 << 1 (0101 << 1) | 10 (1010) |
>> | right shift | 5 >> 1 (0101 >> 1) | 2 (10) |
>>> | zero fill right shift | 5 >>> 1 (0101 >>> 1) | 2 (10) |
Arithmetic
a * (b + c) // grouping
person.age // member
person[age] // member
!(a == b) // logical not
a != b // not equal
typeof a // type (number, object, function...)
x << 2 x >> 3 // minary shifting
a = b // assignment
a == b // equals
a != b // unequal
a === b // strict equal
a !== b // strict unequal
a < b a > b // less and greater than
a <= b a >= b // less or equal, greater or eq
a += b // a = a + b (works with - * %...)
a && b // logical and
a || b // logical or
Arrays≡
var dogs = ["Bulldog", "Beagle", "Labrador"];
var dogs = new Array("Bulldog", "Beagle", "Labrador"); // declaration
alert(dogs[1]); // access value at index, first item being [0]
dogs[0] = "Bull Terier"; // change the first item
for (var i = 0; i < dogs.length; i++) { // parsing with array.length
console.log(dogs[i]);
}
Methods
dogs.toString(); // convert to string: results "Bulldog,Beagle,Labrador"
dogs.join(" * "); // join: "Bulldog * Beagle * Labrador"
dogs.pop(); // remove last element
dogs.push("Chihuahua"); // add new element to the end
dogs[dogs.length] = "Chihuahua"; // the same as push
dogs.shift(); // remove first element
dogs.unshift("Chihuahua"); // add new element to the beginning
delete dogs[0]; // change element to undefined (not recommended)
dogs.splice(2, 0, "Pug", "Boxer"); // add elements (where, how many to remove, element list)
var animals = dogs.concat(cats,birds); // join two arrays (dogs followed by cats and birds)
dogs.slice(1,4); // elements from [1] to [4-1]
dogs.sort(); // sort string alphabetically
dogs.reverse(); // sort string in descending order
x.sort(function(a, b){return a - b}); // numeric sort
x.sort(function(a, b){return b - a}); // numeric descending sort
highest = x[0]; // first item in sorted array is the lowest (or highest) value
x.sort(function(a, b){return 0.5 - Math.random()}); // random order sort
concat, copyWithin, every, fill, filter, find, findIndex, forEach, indexOf, isArray, join, lastIndexOf, map, pop, push, reduce, reduceRight, reverse, shift, slice, some, sort, splice, toString, unshift, valueOf
Strings⊗
var abc = "abcdefghijklmnopqrstuvwxyz";
var esc = 'I don\'t \n know'; // \n new line
var len = abc.length; // string length
abc.indexOf("lmno"); // find substring, -1 if doesn't contain
abc.lastIndexOf("lmno"); // last occurance
abc.slice(3, 6); // cuts out "def", negative values count from behind
abc.replace("abc","123"); // find and replace, takes regular expressions
abc.toUpperCase(); // convert to upper case
abc.toLowerCase(); // convert to lower case
abc.concat(" ", str2); // abc + " " + str2
abc.charAt(2); // character at index: "c"
abc[2]; // unsafe, abc[2] = "C" doesn't work
abc.charCodeAt(2); // character code at index: "c" -> 99
abc.split(","); // splitting a string on commas gives an array
abc.split(""); // splitting on characters
128.toString(16); // number to hex(16), octal (8) or binary (2)
Dates📆
Fri Dec 23 2022 01:58:23 GMT-0700 (Mountain Standard Time)
var d = new Date();
1671785903293 miliseconds passed since 1970
Number(d)
Date("2017-06-23"); // date declaration
Date("2017"); // is set to Jan 01
Date("2017-06-23T12:00:00-09:45"); // date - time YYYY-MM-DDTHH:MM:SSZ
Date("June 23 2017"); // long date format
Date("Jun 23 2017 07:45:00 GMT+0100 (Tokyo Time)"); // time zone
Get Times
var d = new Date();
a = d.getDay(); // getting the weekday
getDate(); // day as a number (1-31)
getDay(); // weekday as a number (0-6)
getFullYear(); // four digit year (yyyy)
getHours(); // hour (0-23)
getMilliseconds(); // milliseconds (0-999)
getMinutes(); // minutes (0-59)
getMonth(); // month (0-11)
getSeconds(); // seconds (0-59)
getTime(); // milliseconds since 1970
Setting part of a date
var d = new Date();
d.setDate(d.getDate() + 7); // adds a week to a date
setDate(); // day as a number (1-31)
setFullYear(); // year (optionally month and day)
setHours(); // hour (0-23)
setMilliseconds(); // milliseconds (0-999)
setMinutes(); // minutes (0-59)
setMonth(); // month (0-11)
setSeconds(); // seconds (0-59)
setTime(); // milliseconds since 1970)
JSONj
var str = '{"names":[' + // crate JSON object
'{"first":"Hakuna","lastN":"Matata" },' +
'{"first":"Jane","lastN":"Doe" },' +
'{"first":"Air","last":"Jordan" }]}';
obj = JSON.parse(str); // parse
document.write(obj.names[1].first); // access
Send
var myObj = { "name":"Jane", "age":18, "city":"Chicago" }; // create object
var myJSON = JSON.stringify(myObj); // stringify
window.location = "demo.php?x=" + myJSON; // send to php
Storing and retrieving
myObj = { "name":"Jane", "age":18, "city":"Chicago" };
myJSON = JSON.stringify(myObj); // storing data
localStorage.setItem("testJSON", myJSON);
text = localStorage.getItem("testJSON"); // retrieving data
obj = JSON.parse(text);
document.write(obj.name);
Loops↶
For Loop
for (var i = 0; i < 10; i++) {
document.write(i + ": " + i*3 + "<br />");
}
var sum = 0;
for (var i = 0; i < a.length; i++) {
sum + = a[i];
} // parsing an array
html = "";
for (var i of custOrder) {
html += "<li>" + i + "</li>";
}
While Loop
var i = 1; // initialize
while (i < 100) { // enters the cycle if statement is true
i *= 2; // increment to avoid infinite loop
document.write(i + ", "); // output
}
Do While Loop
var i = 1; // initialize
do { // enters cycle at least once
i *= 2; // increment to avoid infinite loop
document.write(i + ", "); // output
} while (i < 100) // repeats cycle if statement is true at the end
Break
for (var i = 0; i < 10; i++) {
if (i == 5) { break; } // stops and exits the cycle
document.write(i + ", "); // last output number is 4
}
Continue
for (var i = 0; i < 10; i++) {
if (i == 5) { continue; } // skips the rest of the cycle
document.write(i + ", "); // skips 5
}
Events🕖
<button onclick="myFunction();">
Click here
</button>
Mouse
onclick, oncontextmenu, ondblclick, onmousedown, onmouseenter, onmouseleave, onmousemove, onmouseover, onmouseout, onmouseup
Keyboard
onkeydown, onkeypress, onkeyup
Frame
onabort, onbeforeunload, onerror, onhashchange, onload, onpageshow, onpagehide, onresize, onscroll, onunload
Form
onblur, onchange, onfocus, onfocusin, onfocusout, oninput, oninvalid, onreset, onsearch, onselect, onsubmit
Drag
ondrag, ondragend, ondragenter, ondragleave, ondragover, ondragstart, ondrop
Clipboard
oncopy, oncut, onpaste
Media
onabort, oncanplay, oncanplaythrough, ondurationchange, onended, onerror, onloadeddata, onloadedmetadata, onloadstart, onpause, onplay, onplaying, onprogress, onratechange, onseeked, onseeking, onstalled, onsuspend, ontimeupdate, onvolumechange, onwaiting
Animation
animationend, animationiteration, animationstart
Miscellaneous
transitionend, onmessage, onmousewheel, ononline, onoffline, onpopstate, onshow, onstorage, ontoggle, onwheel, ontouchcancel, ontouchend, ontouchmove, ontouchstart
If - Else⇵
if ((age >= 14) && (age < 19)) { // logical condition
status = "Eligible."; // executed if condition is true
} else { // else block is optional
status = "Not eligible."; // executed if condition is false
}
Switch Statement
switch (new Date().getDay()) { // input is current day
case 6: // if (day == 6)
text = "Saturday";
break;
case 0: // if (day == 0)
text = "Sunday";
break;
default: // else...
text = "Whatever";
}
Data Typesℜ
var age = 18; // number
var name = "Jane"; // string
var name = {first:"Jane", last:"Doe"}; // object
var truth = false; // boolean
var sheets = ["HTML","CSS","JS"]; // array
var a; typeof a; // undefined
var a = null; // value null
Objects
var student = { // object name
firstName:"Jane", // list of properties and values
lastName:"Doe",
age:18,
height:170,
fullName : function() { // object function
return this.firstName + " " + this.lastName;
}
};
student.age = 19; // setting value
student[age]++; // incrementing
name = student.fullName(); // call object function
Numbers and Math∑
var pi = 3.141;
pi.toFixed(0); // returns 3
pi.toFixed(2); // returns 3.14 - for working with money
pi.toPrecision(2) // returns 3.1
pi.valueOf(); // returns number
Number(true); // converts to number
Number(new Date()) // number of milliseconds since 1970
parseInt("3 months"); // returns the first number: 3
parseFloat("3.5 days"); // returns 3.5
Number.MAX_VALUE // largest possible JS number
Number.MIN_VALUE // smallest possible JS number
Number.NEGATIVE_INFINITY // -Infinity
Number.POSITIVE_INFINITY // Infinity
Math.
var pi = Math.PI; // 3.141592653589793
Math.round(4.4); // = 4 - rounded
Math.round(4.5); // = 5
Math.pow(2,8); // = 256 - 2 to the power of 8
Math.sqrt(49); // = 7 - square root
Math.abs(-3.14); // = 3.14 - absolute, positive value
Math.ceil(3.14); // = 4 - rounded up
Math.floor(3.99); // = 3 - rounded down
Math.sin(0); // = 0 - sine
Math.cos(Math.PI); // OTHERS: tan,atan,asin,acos,
Math.min(0, 3, -2, 2); // = -2 - the lowest value
Math.max(0, 3, -2, 2); // = 3 - the highest value
Math.log(1); // = 0 natural logarithm
Math.exp(1); // = 2.7182pow(E,x)
Math.random(); // random number between 0 and 1
Math.floor(Math.random() * 5) + 1; // random integer, from 1 to 5
Constants like Math.PI:
E, PI, SQRT2, SQRT1_2, LN2, LN10, LOG2E, Log10E
Regular Expressions\n
var a = str.search(/CheatSheet/i);
Modifiers
i perform case-insensitive matching
g perform a global match
m perform multiline matching
Patterns
\ Escape character
\d find a digit
\s find a whitespace character
\b find match at beginning or end of a word
n+ contains at least one n
n* contains zero or more occurrences of n
n? contains zero or one occurrences of n
^ Start of string
$ End of string
\uxxxx find the Unicode character
. Any single character
(a|b) a or b
(...) Group section
[abc] In range (a, b or c)
[0-9] any of the digits between the brackets
[^abc] Not in range
\s White space
a? Zero or one of a
a* Zero or more of a
a*? Zero or more, ungreedy
a+ One or more of a
a+? One or more, ungreedy
a{2} Exactly 2 of a
a{2,} 2 or more of a
a{,5} Up to 5 of a
a{2,5} 2 to 5 of a
a{2,5}? 2 to 5 of a, ungreedy
[:punct:] Any punctuation symbol
[:space:] Any space character
[:blank:] Space or tab
PromisesÞ
function sum (a, b) {
return Promise(function (resolve, reject) {
setTimeout(function () { // send the response after 1 second
if (typeof a !== "number" || typeof b !== "number") { // testing input types
return reject(new TypeError("Inputs must be numbers"));
}
resolve(a + b);
}, 1000);
});
}
var myPromise = sum(10, 5);
myPromsise.then(function (result) {
document.write(" 10 + 5: ", result);
return sum(null, "foo"); // Invalid data and return another promise
}).then(function () { // Won't be called because of the error
}).catch(function (err) { // The catch handler is called instead, after another second
console.error(err); // => Please provide two numbers to sum.
});
States
pending, fulfilled, rejected
Properties
Promise.length, Promise.prototype
Methods
Promise.all(iterable), Promise.race(iterable), Promise.reject(reason), Promise.resolve(value)
Global Functions()
eval(); // executes a string as if it was script code
String(23); // return string from number
(23).toString(); // return string from number
Number("23"); // return number from string
decodeURI(enc); // decode URI. Result: "my page.asp"
encodeURI(uri); // encode URI. Result: "my%page.asp"
decodeURIComponent(enc); // decode a URI component
encodeURIComponent(uri); // encode a URI component
isFinite(); // is variable a finite, legal number
isNaN(); // is variable an illegal number
parseFloat(); // returns floating point number of string
parseInt(); // parses a string and returns an integer
Errors⚠
try { // block of code to try
undefinedFunction();
}
catch(err) { // block to handle errors
console.log(err.message);
}
Throw error
throw "My error message"; // throw a text
Input validation
var x = document.getElementById("mynum").value; // get input value
try {
if(x == "") throw "empty"; // error cases
if(isNaN(x)) throw "not a number";
x = Number(x);
if(x > 10) throw "too high";
}
catch(err) { // if there's an error
document.write("Input is " + err); // output error
console.error(err); // write the error in console
}
finally {
document.write("</br />Done"); // executed regardless of the try / catch result
}
Error name values
RangeError A number is "out of range"
ReferenceError An illegal reference has occurred
SyntaxError A syntax error has occurred
TypeError A type error has occurred
URIError An encodeURI() error has occurred
Source:
https://htmlcheatsheet.com/js/
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