Conditionals

Comparisions

Conditional Statements

• if

  if (hour < 18) {  
  	greeting = "Good day";  
  }
  

• else

  if (hour < 18) {
  	greeting = "Good day";
  } else {
  	greeting = "Good evening";
  }
  

• else if

  if (time < 10) {  
  	greeting = "Good morning";  
  } else if (time < 20) {  
  	greeting = "Good day";  
  } else {  
  	greeting = "Good evening";  
  }
  

Logical operators

• || (OR)

  • There are four possible logical combinations:

    alert( true || true );   // true
    alert( false || true );  // true
    alert( true || false );  // true
    alert( false || false ); // false
    

  • In other words, OR returns the first truthy value it finds.

• && (AND)

  • AND returns true if both operands are truthy and false otherwise.
  • Given multiple AND’ed values, the AND && operator does the following:
    1. Evaluates operands from left to right.
    2. For each operand, converts it to a boolean. If the result is false, stops and returns the original value of that operand.
    3. If all operands have been evaluated (i.e. all were truthy), returns the last operand.
  • In other words, AND returns the first falsy value or the last value if none were found in case of multiple AND’ed.
  • We can also pass several values in a row. See how the first falsy one is returned:

    alert( 1 && 2 && null && 3 ); // null
    

  • When all values are truthy, the last value is returned:

    alert( 1 && 2 && 3 ); // 3, the last one
    

  • Precedence of AND && is higher than OR ||.

• ! (NOT)

  • The operator accepts a single argument and does the following:
    1. Converts the operand to boolean type: true/false.
    2. Returns the inverse value.

    alert( !true ); // false
    alert( !0 ); // true
    

  • A double NOT !! is sometimes used for converting a value to boolean type. That is, the first NOT converts the value to boolean and returns the inverse, and the second NOT inverses it again. In the end, we have a plain value-to-boolean conversion.

    alert( !!"non-empty string" ); // true
    alert( !!null ); // false
    

  • We can use a built-in Boolean function to do the same.

Ternary operator

• condition ? run this code : run this code instead

•   const greeting = isBirthday
    ? "Happy birthday Mrs. Smith — we hope you have a great day!"
    : "Good morning Mrs. Smith.";
  

• Here we have a variable called isBirthday — if this is true, we give our guest a happy birthday message; if not, we give her the standard daily greeting.

Switch

•   switch (expression) {
    	case x:
    		// execute case x code block
    		break;
    	case y:
    		// execute case y code block
    		break;
    	default:
    		// execute default code block
    }