Branching in JavaScript

1. if-if else-else

The most common method to set part of your code under specific condition is to use if-else , where if part will be called if conditions are met and else if not. Also we can use else if combination, when we have several part of code that 100% sure shouldn't be called at the same time.

const a = true;
const b = true;
const c = false;

// Output: This log will be printed in console
if (a) {
  console.log("This log will be printed in console");
}

// Output: This one will be printed instead
if (!b) {
  console.log("This won't");
} else {
  console.log("This one will be printed instead");
}

// Output: This one will be displayed
if (a && c) {
  console.log("This one also will be ignored");
} else if (a && b) {
  console.log("This one will be displayed");
} 
// Btw, we can continue if-else if series as long as we want, and even add else for case, when no conditions met

2. Ternary (conditional) operator

This operator has the following form: condition ? ifTrue : ifFalse , where first part is condition, pretty same as we used for if-else , second - if condition met, and the third one - if not.

const a = true;
const b = false;

// Output: Thats true
console.log(a ? 'Thats true' : 'Thats false');

// Output: Thats false
console.log(b ? 'Thats true' : 'Thats false');

// You can also use nested ternary operator, but I not recomment to do so, because it will make your code less readable
// Output: Thats true in false
console.log(b ? 'Thats true' : a ? 'Thats true in false' : 'Just false');

3. && and || operators

This one is a bit more trickier.

Operator || will return first value that was considered as true (non-empty string, not zero number, object, array, true), or if there is no such value, it will return last false value (false, empty string, number 0, null, undefined).

const a = null || 'its a';
const b = 'its b' || null;
const c = 'its c' || 'its d';
const d = undefined || null;

// Output: its a
console.log(a);
// Output: its b
console.log(b);
// Output: its c
console.log(c);
// Output: null
console.log(d);

Operator && will check values one by one and return the last value if all conditions met (or all values considered as true), or first value that was considered as false.

const a = null && 'its a';
const b = 'its b' && null;
const c = 'its c' && 'its d';
const d = undefined && null;

// Output: null
console.log(a);
// Output: null
console.log(b);
// Output: its d
console.log(c);
// Output: undefined 
console.log(d);

4. Switch statement 

This one uses expression that was set in switch part, and evaluate it with statements placed in case. If it doesn't match any statement, default part called.

const tea = 'green';
switch(tea) {
  case 'black':
    console.log('black tea');
    break; // need to break each time to prevent calling another blocks of code that doesn't even match
  case 'green':
    console.log('green tea');
    break; 
  case 'oolong':
    console.log('oolong tea');
    break; 
  default:
    console.log('no such tea');
    break; 
};