notes.md

Javascript Notes

Object types

Arrays

• good for manipulating data

Sets

• unique values
• good for removing duplicates from array.
• does not have keys or indexes
• Better performance

Objects

• easy to write and access values with. Can use functions as values.

Maps

• keys can have any data type, easy to iterate, easy to compute size, better performance

Higher order function receives another function as an argument

    functionName(‘argument’, callbackFunction);

Arrow function with parameters and callback

    const funcName = arg1 => arg2 => callback;

Bind() and Call()

• method.bind(object) binds “this” to target object

• method.call(object) calls on another object to change “this” declaration

immediately invoked functions

    (function () {
        console.log('This will never run again');
    })();

OR

    () => arrow function

Function Closures

• Function stored in object runs once and remembers variables inside
• Called from outside (global), the variables are passed into global object

    const closureObject = function(){	
        return function(){
            do something
        } 
    }

    const globalObj = closureObj();

Arrays

Slice

    array.slice(startIndex, endIndex);

• Does not modify original array

Splice

    array.splice(index, deleteCount)

• Removes entries after the index
• Modifies original array.

Spread operator

    [...array]

reverse

    array.reverse();

• Modifies original array

concat

    array.concat(array2);

• Combines one array with another (argument)
• Modifies original array
• Can also use [...array1, ...array2] << Does not modify original

join

    array.join('char');

at

    array.at(index)

• ES2022 standard
• works on strings
• Can be used to extract array index from either beginning or end
• Used as alternative to [index]
• Good for method chaining

for

    for (const iterable of array){
        do something
    }

• will get index along with entries:

    for (const [i, iterable] of array.entries())

forEach

    array.forEach(function(iterable){
        do something
    })

    array.forEach(function(entryName[, index, array])){
        do something
    }

• You cannot break out of a forEach loop

Map

    arr.map(function(element, index, array){
        return (doing something to element);
    })

• performs an action on each array value

Filter

    arr.filter(function(iterable){
        return filter condition;
    })

• Passes elements that match condition into new array

Reduce

    arr.reduce(function (accumulator, current, index, arr) {
        return accumulator + current;
    }, [initial value of accumulator]);

• reduce elements to one single value
• Does not create a new array

Chaining

    arr.method().method().method();

• Only works if the first method returns an array

Math

abs([number])

• Gets absolute value (removes negative values)

Ternary operator

    (obj > value ? [do this] : [or do this])

Arrow notation

const obj = (arg1 [,arg2] => [what to return]);

• do not need to use 'return' and leave off semicolon after return