JavaScript Fundamentals Cheat Sheet

Error Handling

Throw Statement

• throw statement will stop execution of the script, unless you handle it

throw new Error('something went wrong')

Error Object

const myError = new Error('This is the error message');
console.log(myError.name); // Error
console.log(myError.message); // This is the error message

• Error instances have name and message properties.

// this:
const x = Error('I was created using a function call!');
​​​​// has the same functionality as this:
const y = new Error('I was constructed via the "new" keyword!');

• When Error is used like a function -- without new, it will return an Error object. Therefore, a mere call to Error will produce the same output that constructing an Error object via the new keyword would

Try...Catch...Finally Statement

function f() {
  try {
    console.log(0);
    throw 'bogus';
  } catch(e) {
    console.log(1);
    return true; // this return statement is suspended
                 // until finally block has completed
    console.log(2); // not reachable
  } finally {
    console.log(3);
    return false; // overwrites the previous "return"
    console.log(4); // not reachable
  }
  // "return false" is executed now  
  console.log(5); // not reachable
}
console.log(f()); // 0, 1, 3, false

• When logging errors to the console inside a catch block, using console.error() rather than console.log() is advised for debugging as it formats the message as an error and adds it to the list of error messages generated by the page
• The finally block executes whether or not an exception is thrown.
• If the finally block returns a value, this value becomes the return value of the entire try-catch-finally production, regardless of any return statements in the try and catch blocks
• Overwriting of return values by the finally block also applies to exceptions thrown or re-thrown inside of the catch block

Getters / Setters

const currentYear = new Date().getFullYear();

let car = {
    yearPurchased : 2001,

    get yearsOwned() {
        return currentYear - this.yearPurchased;
    },

    set yearsOwned(years) {
        this.yearPurchased = currentYear - years;
    }
}

console.log(car.yearPurchased);
console.log(car.yearsOwned);

car.yearsOwned = 5;

console.log(car.yearPurchased);

ES6 Features

let person = {
    firstName : "Randy",
    lastName: "Beltran",

    get fullName() {
        return `${this.firstName} ${this.lastName}` // String Literal
    },

    set fullName(name) {
        [this.firstName, this.lastName] = name.split(' '); // Destructuring Assignment
    }
}

console.log(person.fullName);

person.fullName = "Jason Humphrey";

console.log(person.firstName);
console.log(person.lastName);


// Use of Promises
const myPromise = new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve('Data was returned');
    }, 3000);
});

myPromise
    .then(data => {
        console.log(data);
    });

Sync vs Async

console.log('Start!');

const myPromise = new Promise(function(resolve, reject) {
    setTimeout(function() {
        resolve('Data was returned');
    }, 3000);
});

myPromise
    .then(function(data) {
        console.log(data);
    })

console.log('End!');

Scope and Closure

function closureStart() {
    var topic = 'closure';

    function display() {
        alert(topic);
    }

    return display;
}

var myClosure = closureStart();

myClosure();

Arrays and Objects

let myString = 'Hello';
let myString2  = "Hello World";
let myString3 = `Hi ${myString}`;

let myNum = 4;

let isGood = true;

let myShoppingList = ['Flour', 'Sugar', 'Eggs'];
let myShoppingList2 = {
  0: 'Flour',
  1: 'Sugar',
  2: 'Eggs'
};
let myInformation = {
  street: 'Main St',
  phone: '(555) 555-5555',
  state: 'CA'
};
let myPoints = [
  {
    x: 1,
    y:1
  },
  
];
let myPoints2 = [
  [1,1]
];

let myFriendsList = [
  {
    name: 'Sally',
    list: [{
      product: 'Cake',
      brand: 'Betty Crocker'
    }]
  }
];

let storeCustomers = {
  Walmart: myFriendsList
  
};

let scores = {
  2009: [89, 79]
};
let scores2 = [[89, 79], [88, 78]];