Types and globals
Comments
// Single comment
/* Multi-line comment. Make sure this is the correct
commenting style.
*/
Essentials
let name = "My name";
name = "Rename";
let and const have block scope, var has global or function scope.
Strings
let doubleQuote = "Don't you dare do that"
let name = "Jimmy"
// template strings
let sentence = `My name is ${name}`;
// Combine strings
s1.concat(s2)
// split string into array by a sep
// s.split("sep")
let s = "This is going to be an array"
let arr = s.split(" ")
console.log(arr);
Output:
[
'This', 'is',
'going', 'to',
'be', 'an',
'array'
]
join
Convert array to string:
let letters = ["s", "t", "r", "i", "n", "g"]
let word = letters.join("")
let comma_word = letters.join()
console.log("word:", word);
console.log("comma_word:", comma_word);
Output:
word: string
comma_word: s,t,r,i,n,g
indexOf and lastIndexOf
Find the index of a letter or word in a string with indexOf() (or lastIndexOf):
let word = "Javascript"
let r = word.indexOf("r")
let x = word.indexOf("x")
console.log(r);
console.log(x);
If the letter is not in the string, then it returns a -1:
Output:
6
-1
If you want to use a regex, use the search() method. It behaves exactly as indexOf(). Note that indexOf() is faster than search.
charAt
charAt finds the character at the specified index:
let line = "To be, or not to be"
let c = line.charAt(1)
console.log(c); // o
Output:
o
slice
slice(start, end) can create a new string from an existing string–it does not alter the existing string. If you omit end, it slices from start to the end of the string.
start is inclusive. end is uninclusive:
let s = "Mississippi"
let sub1 = s.slice(2, 4)
let sub2 = s.slice(5, 7)
let sub3 = s.slice(4)
console.log(sub1); // iss
console.log(sub2); // ss
console.log(sub3); // issippi
replace
replace(old, new) changes only the first occurence, and replaceAll(old, new) replace every occurence:
let old = "This oh is oh my hello to oh you doh!"
let rep = old.replace("oh", "x")
let repAll = old.replaceAll("oh", "x")
console.log(rep); // This x is oh my hello to oh you doh!
console.log(repAll); // This x is x my hello to x you dx!
toUpperCase and toLowerCase
toUpperCase and toLowerCase:
let lower = "abcdefghijklmnopqrstuvwxyz"
let upper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
let lowToUpper = lower.toUpperCase();
let upToLower = upper.toLowerCase();
console.log(lowToUpper); // ABCDEFGHIJKLMNOPQRSTUVWXYZ
console.log(upToLower); // abcdefghijklmnopqrstuvwxyz
startsWith and endsWith
startsWith(word) and endsWith(word) return a boolean that indicates whether the string starts or ends with word. Note that it is case-sensitive, and punctuation counts as part of the string:
let s = "The string is the thing."
let starts = s.startsWith("the")
let ends = s.endsWith("thing.")
console.log(starts); // false
console.log(ends); // true
Numbers
let intNr = 1;
let decNr = 1.5;
let expNr = 1.4e15;
let octNr = 0o10; //decimal version would be 8
let hexNr = 0x3E8; //decimal version would be 1000
let binNr = 0b101; //decimal version would be 5
Global functions
Many of these are global functions:
isNan(x) // is x a number, returns boolean
isInteger(x) // is x a Number
let x = 1.2345
let twoDec = x.toFixed(2) // 1.23
let precision = x.toPrecision(2) // 1.2
Boolean
Use true or false.
Data types
let str = "Hello";
let nr = 7;
let bigNr = 12345678901234n;
let bool = true;
let sym = Symbol("unique");
let undef = undefined;
let unknown = null;
console.log("str", typeof str);
console.log("nr", typeof nr);
console.log("bigNr", typeof bigNr);
console.log("bool", typeof bool);
console.log("sym", typeof sym);
console.log("undef", typeof undef);
console.log("unknown", typeof unknown);
Converting (casting)
String(val)
Number(val)
Boolean(val)
Equality
The double equals (==) checks for equal value, not data type. The triple equals (===) checks both value and data type:
let x = 5
let y = '5'
x == y
// true
x === y
// false
URI and URL encoding
URLs use percent encoding. For example, a space is encoded as %20:
let uri = "https://www.example.com/submit?name=rick randolph";
console.log("URI:", uri);
let encoded_uri = encodeURI(uri)
console.log("Encoded:", encoded_uri);
let decoded_uri = decodeURI(encoded_uri)
console.log("Decoded:", decoded_uri);
Output:
URI: https://www.example.com/submit?name=rick randolph
Encoded: https://www.example.com/submit?name=rick%20randolph
Decoded: https://www.example.com/submit?name=rick randolph
Math
let highest = Math.max(2, 56, 12, 1, 233, 4); // 233
let lowest = Math.min(2, 56, 12, 1, 233, 4); // 1
let result = Math.sqrt(64); // 8
let result2 = Math.pow(5, 3); // 5^3 = 125
let x = Math.round(6.4); // 6
let y = Math.round(8.8); // 9
let x = Math.ceil(6.4); // 7
let y = Math.ceil(8.8); // 9
let x = Math.floor(6.4); // 6
let y = Math.floor(8.8); // 8
Date
Create dates with the built-in Date object and its methods:
let today = new Date() // 2023-03-09T15:45:44.414Z
let secsSinceUnixEpoch = Date.now() // 1678376744419
let unixEpochPlus1000 = new Date(1000) // 1970-01-01T00:00:01.000Z
let stringDate = new Date("Thurs Mar 09 2023 10:43:00 GMT+0200"); // 2023-03-09T08:43:00.000Z
let specificDate = new Date(2023, 2, 9, 10, 45, 0, 0); // 2023-03-09T15:45:00.000Z
/*** accepted formats ***/
// new Date(year, monthIndex)
// new Date(year, monthIndex, day)
// new Date(year, monthIndex, day, hours)
// new Date(year, monthIndex, day, hours, minutes)
// new Date(year, monthIndex, day, hours, minutes, seconds)
// new Date(year, monthIndex, day, hours, minutes, seconds, milliseconds)
Get and set date elements:
let d = new Date();
console.log("Day of week:", d.getDay()); // Day of week: 4
console.log("Day of month:", d.getDate()); // Day of month: 9
console.log("Month:", d.getMonth()); // Month: 2
console.log("Year:", d.getFullYear()); // Year: 2023
console.log("Seconds:", d.getSeconds()); // Seconds: 15
console.log("Milliseconds:", d.getMilliseconds()); // Milliseconds: 672
console.log("Time:", d.getTime()); // Time: 1678377375672
// change these methods to set* to change the date:
d.setFullYear(2040) // 2040-03-09T15:58:22.622Z
Parse dates
parse() can parse strings into epoch dates.
let d1 = Date.parse("June 5, 2021"); // 1622851200000
let d2 = Date.parse("6/5/2021"); // 1622851200000
Date to string
toDateString() and toLocaleDateString() convert an epoch date into a string:
let d = new Date();
console.log(d.toDateString()); // Thu Mar 09 2023
console.log(d.toLocaleDateString()); // 3/9/2023