Lecture2.js

// Operations with strings
let firstPhrase = 'Hello world!';

console.log(`My first phrase in JavaScript is "${firstPhrase}"`);
// .length property returns the length of a string
console.log("The length of this phrase is", firstPhrase.length, "symbols");
// typeof operator returns data type
console.log("The type of this variable is", typeof firstPhrase);
console.log(`The 0 index of this phrase is "${firstPhrase[0]}"`);
console.log(`The 6 index of this phrase is "${firstPhrase[6]}"`);
// .toLowerCase() method returns converts all the characters in a string to lowercase letters
console.log(`Modified phrase to lower case is "${firstPhrase.toLowerCase()}"`);
// .toUpperCase() method returns converts all the characters in a string to uppercase letters
console.log(`Modified phrase to upper case is "${firstPhrase.toUpperCase()}"`);
// printing character from ASCII table by its' code
console.log(String.fromCharCode(63));
// trim extra spaces, new lines, tabs from both sides of a string. Doesn't change the original string!
let textToTrim = "   I'm learning JavaScript! \n	";
console.log(textToTrim); // before trim
console.log(textToTrim.trim()); // after trim
// slice string
console.log(firstPhrase.slice(0, 5)); // slices from firstPhrase[0] to 5th symbol (firstPhrase[4])
console.log(firstPhrase.slice(3));// slices from firstPhrase[3] to the end
// slice and trim string
console.log(textToTrim.slice(16).trim());
// variable.replace(what we want to replace, what should be instead)
console.log(firstPhrase.replace('world', 'Planet'));
// 'string'.replace(what we want to replace, what should be instead)
console.log('google'.replace('go', 'mo'));
// variable.repeat(number of repeats) - how many times we want our string to be repeated
console.log(firstPhrase.repeat(2));
// 'string'.repeat(number of repeats) - how many times we want our string to be repeated
console.log("haha".repeat(2));
// 'string'.startswith() checking if our string starts with specific symbol - returns true
console.log("haha!".startsWith('h'));
// 'string'.endswith() checking if our string ends with specific symbol - returns true
console.log("haha!".endsWith('!'));


// Math operators - now in a separate file called mathOperators.js

// Comparison operators
// ==, ===, !=, !==, >, <, >=, <=

let c = 5;
let bool;
console.log(`c is number ${c}`);
bool = (c == 5);
console.log("Is c equal to 5? The answer is", bool); // using not strict operator '==' that compare only values
bool = (c == '5');
console.log("Is c equal to '5'? The answer is", bool); // using not strict operator '==' that compare only values
bool = (c === 5);
console.log("Is c equal to 5? The answer is", bool); // using strict operator '===' - compare values AND data types
bool = (c === '5');
console.log("Is c equal to '5'? The answer is", bool); // using strict operator '===' - compare values AND data types

let d = "5";
let e = -10;
let f = "c";
let g = "C";

console.log(`Is ${d} equal to 5? The answer is ${d == 5}`); // using not strict operator
console.log(`Is ${d} equal to 5? The answer is ${d === 5}`); // using strict operator
console.log(`Is ${e} NOT equal to "-10"? The answer is ${e != "-10"}`); // using not strict operator
console.log(`Is ${e} NOT equal to "-10"? The answer is ${e !== "-10"}`); // using strict operator
console.log(`Is ${f} equal to ${g}? The answer is ${f == g}`); // using not strict operator
console.log(`Is ${f} equal to "c"? The answer is ${f === "c"}`); // using strict operator

let h = 3;
let k = 7;

console.log(`h is number ${h}`);
console.log(`k is number ${k}`);
console.log(`Is h greater than 0? The answer is ${h > 0}`); // using > operator
console.log(`Is h less than k? The answer is ${h < k}`); // using < operator
console.log(`Is h greater than 0 or equal to 0? The answer is ${h >= 0}`); // using >= operator
console.log(`Is k less than 0 or equal to 0? The answer is ${k <= 0}`); // using <= operator

// Logical operators
// && (AND), || (OR), ! (NOT)
// The priority of && operator is HIGHER than ||, so && executes earlier

let bodyMass = 70;

console.log(`The body mass is ${bodyMass} kg`);
console.log(`Is body mass more than 50 OR less than 90 kg? The answer is ${bodyMass > 50 || bodyMass < 90}`);
console.log(`Is body mass more than 10 AND less than 50 kg? The answer is ${bodyMass > 10 && bodyMass < 50}`);

// truth tables
console.log(true && true); // true
console.log(true && false); // false
console.log(false && true); // false
console.log(false && false); // false
console.log(true || true); // true
console.log(true || false); // true
console.log(false || true); // true
console.log(false || false); // false

/* Changing data types
- From string to number using +str OR Number(str);
- From number to string using String(num);
*/

// Converting string to number
let newString = "345";

console.log(`The type of ${newString} is ${typeof newString}`);
console.log(newString + 4); // adding new value to the end of the string
console.log(`Changing the type of ${newString} ${typeof newString} to ${typeof +newString} ${newString}`); // changing string to number by adding "+" sign to the string
console.log(`The type of ${newString} is ${typeof newString}`); // checking that our string is still string because we haven't convert it yet
console.log(`Changing the type of ${newString} ${typeof newString} to ${typeof (Number(newString))} ${newString}`); // changing string to number using Number() method
console.log(`The type of ${newString} is ${typeof newString}`); // checking that our string is still string because we haven't convert it yet
// to convert our variable into a new data type we should declare new variable, for example:
let stringToNumber = Number(newString);

console.log(`The type of ${stringToNumber} is ${typeof (stringToNumber)}`);

// Converting number to string
let newNumber = 237;

console.log(`The type of ${newNumber} is ${typeof newNumber}`);
console.log(`Changing the type of ${newNumber} ${typeof newNumber} to ${typeof (String(newNumber))} ${newNumber}`); // changing number to string using String() method
console.log(`The type of ${newNumber} is ${typeof newNumber}`); // checking that our number is still number because we haven't convert it yet
// to convert our variable into a new data type we should declare new variable, for example:
let numberToString1 = String(newNumber);

const numberToString2 = (number) => String(number); // or do this with just a function;

console.log(`The type of ${numberToString1} is ${typeof (numberToString1)}`);

// Another data types

let newVar; // we didn't define a value of newVar, so it's undefined

console.log(`The type of ${newVar} is ${typeof newVar}`);

let newNull = null; // we define null as a value of newNull variable

console.log(`The type of ${newNull} is ${typeof newNull}`); // null is an object data type

// Conditional statements if and else

let herHeight = 168;
let hisHeight = 168;
if (herHeight < hisHeight){ // condition is in parenthesis if (condition){what to do if our condition is true}
	console.log("He is higher than her");
}
else if (herHeight == hisHeight){ // if have one more condition: else if (another condition){what to do if our condition is true}
	console.log("Their height is equal");
}
else { // else {what to do if our condition is false}
	console.log("She is higher than him");
}

// prompt() method

/* I will comment everything here because prompt isn't supported by Node.js
const userAge = Number(prompt("What's your age?"));
 OR
const userAge = +prompt("What's your age?");

const userAge = Number(prompt("What's your age?", 21)); 21 stands for default hint value

Because by default promt() method returns a string, so we need to convert the string into number
to do some operations or conditional statements.
*/