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Valeri Tandilashvili's JavaScript Notes
Results: 62
Concatenate strings and variables
Variable 
myName
will be concatenated with the string
let myName = 'Valeri';
console.log('My name is ' + myName + '.');
The result will be:
My name is Valeri.
Simple Math calculations
Addition of two integers
5+5; // 10
Multiplication of two integers
5*5; // 25
If we add a number to a string, the number will be converted into a string and the result will be a concatenation of the two strings
5+'12'; // 512
String multiplied by an integer
5*'5'; // 25
Multiplication of a string (but not numeric string) and a number
5*'t'; // NaN
The first operation of the expression is addition and then concatenation
5+5+'2'; // 102
Make an object from a string 
function mapString(string) {
  let map = {};
  for (let i = 0; i < string.length; i++) {
    let letter = string[i];
    if (map[letter]) {
      map[letter].push(i);
    } else {
      map[letter] = [i];
    }
  }
  return map;
}
console.log(mapString('blblllaaha'))
How FOR loop works
In the example code below the loop flows through these steps: Setting initial variable value of i to 0 Testing if the loop should be running while i is 0 Running the code block console.log(i) Updating i to be 1 Testing if the loop should be running while i is 1 Running the code block console.log(i) Updating i to be 2 Testing if the loop should be running while i is 2 This means the code will log 0 and 1 to the console. 
for (let i = 0; i < 2; i++) {
   console​.log(i)
}
falsy values
In JavaScript, there are 6 falsy values:
1. false: the boolean value false
2. 0: the number zero
3. '': the empty string, a string with no characters
4. NaN : stands for "Not a Number", usually caused by math errors
5. undefined: a variable's value before it is assigned a value
6. null: a blank value that can be assigned to a variable
Declare a variable that will be use later to return
In many functions, it is useful to declare a variable for the function to later return. For example:
function calculateSum(numberArray) {
    let sum = 0;
    for (let num of numberArray) {
        sum += num;
    }
    return sum;
}
return false if any member is greater than 4
The following function will return false if any number in an array of numbers is greater than 4. Otherwise, the function will return true
function lessThanFive(numbers) {
  for (let number of numbers) {
    if (number > 4) {
      return false;
    }
  }
  return true;
}
Multiple return in a function
It can be useful to have multiple returns in a function declaration as long as only 1 return is expected to run. For example:
function invert(x) {
  if (x === 0) {
     return 'input was zero';
  } else {
    return 1 / x;
  }
}
The function will still work using an if statement instead of an if...else statement
function invert(x) {
  if (x === 0) {
     return 'input was zero';
  }
  return 1 / x;
}
querySelector VS querySelectorAll
Affects only the first element with the specified class 
.slide
document.querySelector('.slide').remove()
Removes all the elements with the specified class
document.querySelectorAll('.slide').remove()
Fetch JSON API
Fetch with promises
async function start() {
  try {
    fetch("https://dog.ceo/api/breeds/list/all").then(function(response) {
      return response.json()
    }).then(function(data) {
      createBreedList(data.message)
    })
  } catch (e) {
    console.log("There was a problem fetching the breed list.")
  }
}
Fetch with 
async
& 
await
keywords
async function start() {
  try {
    const response = await fetch("https://dog.ceo/api/breeds/list/all")
    const data = await response.json()
    createBreedList(data.message)
  } catch (e) {
    console.log("There was a problem fetching the breed list.")
  }
}
Results: 62