Javascript Jems - Active Logic, Truthy and Falsey
Written by Ian Elliot   
Wednesday, 04 September 2013
Article Index
Javascript Jems - Active Logic, Truthy and Falsey
Lazy Evaluation & And
The Ternary Operator

Ternary expression

You may have encountered the JavaScript ternary expression before but always regarded it as something special and not part of the whole logic of the language but seen in the light of the way AND and OR are implemented it makes a lot more sense. The ternary expression is just another example of active logic. That is:

  • AIIB returns the second expression if A is false
  • A&&B returns the second expression if A is true
  • A?B:C returns B if A is true and C if A is false

In terms of a truth table this too is just a logical function but one with three inputs:

 

A B C A?B:C
F F F F
F F T T
F T F F
F T T T
T F F F
T F T F
T T F T
T T T T

 

In this case it is the "active" interpretation of the logic that makes more sense than the "passive" truth table.

We have already noted that the AND && and OR || operators are the equivalent of if statements. That is:

result=A&&B;

and

result=A||B;

are the same as

if(A)
{
 result=B
}else{
 result=A
}

and

if(A)
{
 result=A
}else{
 result=B
}

It is interesting to notice that the ternary expression is also the equivalent of an if..else statement. That is:

result=A?B:C

is the same as

if(A)
{
 result=B

}else{
 result=C
};

 

This also means that the if and the if..else statements are just  logical functions with the truth table given earlier. This is not the way we usually think about them.

 

Active logic and lazy evaluation make a great deal of sense out of some aspects of JavaScript that might otherwise look strange and arbitrary. JavaScript isn't the only language that has a ternary operator and understanding it in terms of active logic makes it seem much more natural. 

 

Related Articles

True but not true       

Dangerous Logic - De Morgan & Programming       

JavaScript Bit Manipulation 

 

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Last Updated ( Thursday, 05 September 2013 )
 
 

   
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