JavaScript Fundamentals 02. JS Conditional Statement PDF

JavaScript pagrindai Justina Balsė Turinys Palyginimo operatoriai; boolean tipas; Šakotieji algoritmai (if-else ir switch); Loginiai operatoriai (AND, OR, NOT) 2 Comparison operators | Lyginimo operatoriai...

JavaScript pagrindai Justina Balsė Turinys Palyginimo operatoriai; boolean tipas; Šakotieji algoritmai (if-else ir switch); Loginiai operatoriai (AND, OR, NOT) 2 Comparison operators | Lyginimo operatoriai 3 Boolean: true and false | 1 // true (correct) All comparison operators return a alert( 2 > 1 ); boolean value: // false (wrong) true – means “yes”, “correct” alert( 2 == 1 ); or “the truth”. false – means “no”, “wrong” // true (correct) or “not the truth”. alert( 2 != 1 ); Justina Balsė 4 Boolean: true and false | 2 let myString1 = "abc"; The algorithm to compare two strings is simple: let myString2 = "bcd"; 1. Compare the first character of both strings. 2. If the first character from the first string is greater (or less) than the other string’s, then the first string console.log(myString1 > myString2); is greater (or less) than the second. We’re done. // false 3. Otherwise, if both strings’ first characters are the same, compare the second characters the same way. console.log(myString1 < myString2); 4. Repeat until the end of either string. // true 5. If both strings end at the same length, then they are equal. Otherwise, the longer string is greater. Justina Balsė 5 Do you know what will be the output of this code? console.log(3 > 2 > 1); Justina Balsė 6 IF let year = prompt('In which year The if(...) statement evaluates a was ECMAScript-2015 specification condition in parentheses and, published?', ''); if the result is true, executes a if (year == 2022) { block of code. alert( 'You are right!' ); } Justina Balsė 7 IF...ELSE let year = prompt('In which year was the ECMAScript-2015 specification published?', ''); if (year == 2015) { alert( 'You guessed it right!' ); } else { // any value except 2015 alert( 'How can you be so wrong?' ); } Justina Balsė 8 IF...ELSE IF let year = prompt('In which year was Sometimes, we’d like to test the ECMAScript-2015 specification published?', ''); several variants of a condition. The else if clause lets us do that. if (year < 2015) { alert( 'Too early...' ); } else if (year > 2015) { alert( 'Too late' ); } else { alert( 'Exactly!' ); } Justina Balsė 9 Conditional operator ‘?’ let accessAllowed; // let result = condition ? value1 : value2; let age = prompt('How old are you?', let accessAllowed = (age > 18) ? true : false; ''); if (age > 18) { accessAllowed = true; } else { accessAllowed = false; } alert(accessAllowed); Justina Balsė 10 Logical operators There are three logical operators in JavaScript: || (OR) && (AND) ! (NOT) Justina Balsė 11 OR || let hour = 9; Most of the time, OR || is used in an if statement to test if any of the if (hour < 10 || hour > 18) { alert( 'The office is closed.' ); given conditions is true. } Justina Balsė 12 AND && let hour = 12; AND returns true if both let minute = 30; operands are truthy and false if (hour == 12 && minute == 30) { otherwise. alert( 'The time is 12:30' ); } Justina Balsė 13 Logical Operators Justina Balsė 14 SWITCH let a = 2 + 2; The switch has one or more case blocks and an switch (a) { optional default. case 3: alert( 'Too small' ); Here the switch starts to compare a from the break; first case variant that is 3. The match fails. case 4: alert( 'Exactly!' ); Then 4. That’s a match, so the execution starts break; case 5: from case 4 until the nearest break. alert( 'Too large' ); break; If there is no break then the execution default: continues with the next case without any alert( "I don't know such values" ); checks. } Justina Balsė 15 Praktika Uždavinių sprendimas konsolėje. 02 - Šakotieji algoritmai (PraktikaEN) 02 - Šakotieji algoritmai (PraktikaLT) Pasitikriname kodą: https://jshint.com/ 16 Functions | Example 17 Falsy values | 1 false A falsy (sometimes written null falsey) value is a value that is undefined considered false when 0 encountered in a Boolean context. -0 NaN "" Justina Balsė 18 Falsy values | 2 var age = 0; if(age){ console.log("Variable is defined.") } else{ console.log("Variable has NOT been defined.") } Justina Balsė 19 Falsy values | 3 var age = 0; if(age || age === 0){ console.log("Variable is defined.") } else{ console.log("Variable has NOT been defined.") } Justina Balsė 20 Falsy values | 4 let city; Console: let defaultCity = "Vilnius"; let myCity = city || defaultCity; // 1 console.log(myCity); // 1 Vilnius city = "Kaunas"; // 2 myCity = city || defaultCity; Kaunas console.log(myCity); // 2 Justina Balsė 21 Logical Operator || OR console.log(true || false); // 1 console.log(false || true); // 2 console.log(false || false || true); // 3 console.log("" || "abc"); // 4 console.log("abc" || ""); // 5 // Boolean("") -> false // Boolean("abc") -> true Justina Balsė 22 Logical Operator || OR console.log(true || false); // 1 1. true console.log(false || true); // 2 console.log(false || false || true); // 3 2. true console.log("" || "abc"); // 4 3. true console.log("abc" || ""); // 5 4. abc // Boolean("") -> false 5. abc // Boolean("abc") -> true Justina Balsė 23 Logical Operator && AND console.log(true && false); // false console.log(false && true); // false console.log(false && false && true); // false console.log("abc" && 10 && false && "abcd"); // false console.log("abc" && 10 && NaN && "abcd"); // NaN console.log("abc" && 10 && "def" && "Hello"); // Hello Justina Balsė 24 Short-circuit evaluation && AND Rezultatas: let isAdmin1 = true; isAdmin1 == true && console.log("Admin1 page"); Admin1 page let isAdmin2 = true; Admin2 page isAdmin2 && console.log("Admin2 page"); Justina Balsė 25 Kartojame Vaizdo įrašas apie Short Circuiting: https://www.youtube.com/watch?v=O4QKfjh8jJM Javascript If Else Statements 26