JavaScript cheat sheet
96 one-line examples for JavaScript. Open it while you code, or print it and keep it near the keyboard. Free, with no signup and no ads.
Variables and Types
| Code | What it does |
|---|---|
let x = 5; | Declare a variable that you can change. |
const y = 10; | Declare a value that cannot change. |
var z = 1; | Old style variable. Prefer let or const. |
let name = "Ana"; | A string holds text inside quotes. |
let ok = true; | A boolean is true or false. |
let n = null; | Null means no value on purpose. |
let u; | An empty variable has value undefined. |
typeof 5 | Return the type name as a string. |
let a = 1, b = 2; | Declare two variables on one line. |
x = 7; | Assign a new value to a variable. |
let big = 10n; | A BigInt holds very large integers. |
let s = Symbol(); | A symbol is a unique identifier. |
Operators
| Code | What it does |
|---|---|
a + b | Add two numbers or join two strings. |
a - b | Subtract the right number from the left. |
a * b | Multiply two numbers. |
a / b | Divide the left number by the right. |
a % b | Return the remainder of a division. |
a ** 2 | Raise a number to a power. |
a === b | Check value and type are equal. |
a !== b | Check value or type is not equal. |
a > b | Check if the left value is larger. |
a && b | True when both sides are true. |
a || b | True when one side is true. |
!a | Reverse a boolean value. |
Strings
| Code | What it does |
|---|---|
s.length | Return the number of characters. |
s.toUpperCase() | Return the string in capital letters. |
s.toLowerCase() | Return the string in small letters. |
s.trim() | Remove spaces from both ends. |
s.includes("ab") | Check if the text contains a part. |
s.indexOf("a") | Return the position of the first match. |
s.slice(0, 3) | Copy a part of the string. |
s.replace("a", "b") | Replace the first match with new text. |
s.split(",") | Split the string into an array. |
s.repeat(3) | Return the string copied three times. |
s.charAt(0) | Return the character at a position. |
`Hi ${name}` | A template string inserts a variable. |
Arrays
| Code | What it does |
|---|---|
let a = [1, 2, 3]; | Create an array with three numbers. |
a.push(4) | Add an item to the end. |
a.pop() | Remove and return the last item. |
a.shift() | Remove and return the first item. |
a.unshift(0) | Add an item to the start. |
a.length | Return the number of items. |
a[0] | Read the item at index zero. |
a.indexOf(2) | Return the position of an item. |
a.join("-") | Join all items into one string. |
a.slice(1, 3) | Copy a part of the array. |
a.includes(2) | Check if the array holds an item. |
a.reverse() | Reverse the order of the items. |
Objects
| Code | What it does |
|---|---|
let o = {a: 1}; | Create an object with one property. |
o.a | Read a property with dot notation. |
o["a"] | Read a property with bracket notation. |
o.b = 2; | Add or change a property. |
delete o.b; | Remove a property from the object. |
"a" in o | Check if a property exists. |
Object.keys(o) | Return an array of property names. |
Object.values(o) | Return an array of property values. |
Object.entries(o) | Return an array of key and value pairs. |
let {a} = o; | Copy a property into a variable. |
let c = {...o}; | Make a shallow copy of the object. |
Object.assign({}, o) | Copy properties into a new object. |
Functions
| Code | What it does |
|---|---|
function f(a) { return a; } | Declare a named function with one parameter. |
const f = (a) => a; | An arrow function returns the value. |
f(3) | Call a function with one argument. |
function f(a = 1) {} | Set a default value for a parameter. |
function f(...r) {} | Collect extra arguments into an array. |
return; | Stop the function and return nothing. |
const f = function () {}; | Store a function in a variable. |
(() => 1)() | Call an arrow function immediately. |
f.call(null, 3) | Call a function with a set this value. |
f.apply(null, [3]) | Call a function with arguments in an array. |
f.bind(null) | Make a new function with a fixed this. |
typeof f === "function" | Check that a value is a function. |
Control Flow
| Code | What it does |
|---|---|
if (a > 1) {} | Run a block when the test is true. |
else {} | Run a block when the test is false. |
a > 1 ? x : y | Pick one value with a short test. |
switch (a) { case 1: } | Compare one value against many cases. |
break; | Leave the switch or loop now. |
for (let i = 0; i < 3; i++) {} | Repeat a block a set number of times. |
while (a < 3) {} | Repeat a block while a test is true. |
do {} while (a < 3); | Run the block once, then test. |
for (const k in o) {} | Loop over the keys of an object. |
for (const v of a) {} | Loop over the values of an array. |
continue; | Skip to the next loop step. |
throw new Error("bad"); | Stop the code and raise an error. |
Array Methods
| Code | What it does |
|---|---|
a.map(x => x * 2) | Make a new array with changed items. |
a.filter(x => x > 1) | Make a new array with matching items. |
a.reduce((s, x) => s + x, 0) | Combine all items into one value. |
a.find(x => x > 1) | Return the first matching item. |
a.findIndex(x => x > 1) | Return the position of the first match. |
a.some(x => x > 1) | Check if any item matches. |
a.every(x => x > 1) | Check if all items match. |
a.sort((x, y) => x - y) | Sort the array with a compare function. |
a.forEach(x => log(x)) | Run a function for each item. |
a.concat([4]) | Join two arrays into a new one. |
a.flat() | Flatten nested arrays by one level. |
Array.isArray(a) | Check that a value is an array. |