Basic Concepts Flashcards
JavaScript is defined as…
A lightweight, multi-paradigm, single-threaded, dynamic scripting language that is found in all modern web technologies
The main 8 types of the JavaScript language
1) Number
2) BigInt
3) String
4) Boolean
5) Object
6) Null
7) Undefined
8) Symbol
How is the Number type implemented
As a double-precision 64-bit binary format IEEE 754 value
What are some subclasses of Objects available to you in JavaScript
- Function
- Array
- Date
- RegExp
- Math
parseInt()
this will parse a string into an integer value, where the first parameter is the string, and the second is the base you want to parse out.
NaN
This means ‘Not a Number’, and is the result of trying to use a non-number value in a number context.
i.e. parseInt(‘hello’, 10);
use Number.isNaN(value) to check for NaNness
NaN is ‘toxic’
Any mathematical operation with a NaN, will result in NaN
parseFloat()
Similar to parseInt, but only uses base 10 and will output the floating point representation of the value parsed.
Falsy Values
False, 0, “”, NaN, null, undefined
Truthy Values
Anything that is not Falsy
let vs. const. vs. var
let: declares a mutable variable in block scope
const: declares an immutable variable in block scope
var: declares a mutable variable with global scope (DONT USE)
for…in vs. for…of
for in is used to iterate over enumerable values. It is good for checking the existence of keys in a map/object. does not guarantee order of enumeration.
for of is useful for iteration of iterable values, so arrays, strings, etc. more often than not, this will be the preferred one.
x && x.name
as long as x exists, return x.name
x.name || x.getName()
if x.name doesn’t exist, get it
A variable can start with
Any letter along with $ and _
??
Nullish Coalescence Operator
a ?? b
if ‘a’ is null/undefined then ‘b’, otherwise ‘a’
== vs. ===
Equality vs. Strict Equality
== : this will attempt to convert the types of the value on each side of the equals sign into the same type before comparing.
===: this will do no such comparison and will strictly compare the two values for equality
comma operator
evaluates everything in the expression, from left to right, and returns the final expression evaluated. (the rightmost value after the final comma)
function (x = 1, x = 2) {}
Functions can be given default parameters, this allows for default values to be used in the event that none are passed in
Transpilers
Special piece of software that translates one source code into another. Main difference between this and a compiler, is there is no speed/performance improvement intended. It would be like translating python into ruby.
ex. of transpilers
Babel
Polyfills
Basically a pollyfill is a function that exists because the actual function you are using is not supported yet in some browsers.
This is a consequence of the language being updated without full support for its updates. So certain functions don’t actually have any meaning in older browsers (ex. Math.trunc()) and have to be substituted with a “fill”
?.
Optional Chaining
Allows you to safely access nested values of an object, even if they, or the object, or a subcall doesn’t exist
it will return undefined in the case where something along the chain does not exist, rather than throwing an error
can also use ?.() for optional method calls and ?.[] for optional array access
Symbols
Guaranteed to be Unique
They are a new addition to the language
“use strict”
Places the execution environment into strict mode.
Makes it easier to write secure JavaScript
This essentially does 3 things
1) Eliminates some JS silent errors by changing them to throw errors
2) Fixes mistakes that make it difficult for JS engines to make optimizations
3) Prohibits some syntax from being used
toFixed(number)
A number method that rounds to a fixed decimal place and returns it as a string
Number, String, Boolean wrappers
These are wrappers that will covert the value passed in into the type of the wrapper. Using new will create an object, but without new, it is just a cast essentially.
1_000_000
The underscore is a valid separator for numbers and will be parsed as if they were not there
Math. floor, ceil, round, trunc
floor: rounds down
ceil: rounds up
round: rounds to nearest integer
trunc: removes anything after the decimal (no rounding)
isFinite(value), isNaN(value)
Infinity and Nan checks
Object.is(a,b)
This is another way to use ===
