javascript Flashcards
how node treat module files
node never process the .js file directly, it wraps the module as a function and pass it to the chrome v8 engine
what’s the global object in node.js
global
what’s the global object in chrome
window; document
naming convention for class
ClassName
same folder path in require; parent folder path in require statement
same folder ./xxx(.js) parent folder ../xxx(.js)
how to load a module
require(‘’)
EMACscript 6 fancy string function is called?
Template String
the node.js built in packages
path os fs http events ( class: EventEmitter; methods: emit & on )
Since events package provides an EventEmitter class (not an instance), what needs to be paid attention when working on the events?
the actual registering event and emit event action need to be based
what makes the function become the special constructor function inside a class defination
give the function name as ‘constructor’, then it will work with the new key word
what the class sytax sugar does
- generating the function that 1) has the name of the class given and 2) coding block as the constructor method - attach all other attributes to the .prototype attribute of the function with class name
is most of the javascript keyword little cap
yes (new, function, class, cnst…)
how to inheriant from another class when defining an class
using ‘extends’ keyword
singular module returning object
module.exports =
is module global?
it is actual a local variable that node.js passed into the wrapped function of the module
how to see the wrapped function of the module
set a syntax error in the very 1st line of a module like ‘var =;’ and then require it somewhere else
how to set async spin over certain time interval
- return a new Promise 2. Inside the promise, define another async function 3. Inside the function, if goal reached, then resolved; if not, use setTimeout to sell the call again. (*** The async function is within the ‘new Promise’, so whenever it resolved, the same promise instance resolved) 4. Call the async function manuallyafter defination
how to set parameter’s default value
constructor ({ keyId, secretKey, paper = true, bucketPct = 0.25 }) { … }
var scope situation
var only socpe to either functional scope or global scope (window or global object)
let and const sope
they scope to the block scope (curly brackets)
what is Hoisting in JS
variable can be used before the definition. (hoisting means moving all declarations to the top of the current scope
what is a primitive type in JS
it is not an object and has no methods
primitive types are immutable, how to approve it
you can not use an string method to alter e.g. toUpperCase the data itself (on the other side, the array method e.g. push can alter the array); the string method can then only manipulate the return value
what is Symbol in JS
Symbols are often used to identify object properties.Often to avoid name clashing between properties, since no symbol is equal to another; No two symbols are the same ‘===’
null vs undefined
null is an object; undefined is a type null can be assigned to variable; undefined appears when nothing assigned to variable
Map vs Object
JS 3 data structues
- Array, list of object
- Map, key-value pair, simplified the historicially used Object
- Set, unquie value of Array list
Date in JS
- number of milisecond since 1970
- new Date() - create a new Date object
- Date.now()
- Date.parse() - parse string date representation
Date - To get Date, Month and Year or Time
let [month, date, year] = new Date().toLocaleDateString(“en-US”).split(“/”) let [hour, minute, second] = new Date().toLocaleTimeString(“en-US”).split(/:| /)
Template String
string text ${expression} string text
Implicity Conversion
Implicit Coercion: Type conversion is done implicitly by JavaScript.
- 12 + “” //Output is “12” as number 12 is converted to string “12”
- “15” * 2 //Output is 30 as string 15 is converted to number 15
- “15” - “11” //Output is 4 as both the strings are converted to number
- undefined + 6 //Output is NaN as undefined could not be converted to number
using for loop through an array
using ‘for … of ..’ to loop through the array
using ‘for…in…’ to loop through object
‘while’ loop
‘while’ loop with ‘break’
‘while’ loop with ‘continue’
loop using ‘do.. while…’
‘for.Each…’ to loop through array
what is setter and getter in js
- let the method can be accessed like a true property from outside the object
Destructuring Assignment Syntax
Accessing the await-ed value inline (JS)?
async
how to inline invoke (async) function
using grouping operator ( )
The grouping operator ( ) controls the precedence of evaluation in expressions.
npm package.json vs. package-lock.json
https://stackoverflow.com/questions/45052520/do-i-need-both-package-lock-json-and-package-json
node.js working with json file
package fs
function JSON.parse()
what is following statement:
$(function () {
// do stuff
});
.then behavior depends on handler function
When passing callback function to setTimeOut, what if the target callback function is a method of an instance
Do not pass the function directly as when the callback function invoked, the scope changes.
put the callback function with ‘this.’ inside another wrapper function and pass the wrapper function as the parameter to setTimeOut.
use destructuring in JavaScript function arguments
yes, this is a bit odd but it makes sense. And the deconstructed arguments can be used directly inside the function (even though it looks like a attribute inside a parameter object but it doesn’t need a top level object to be referred to)
why in node.js, we should always use ‘module.exports = ‘ rather than ‘ exports = ‘ when exporting single object
Because assign value to ‘exports’ directly will change the referenced object it is pointing to but the node js will only export to the object point by module.exports. (They were originally pointing to the same object)
https://stackoverflow.com/questions/7137397/module-exports-vs-exports-in-node-js/26451885#26451885
