Essentials of Javascript - Cultural View

?: 52
?:
In computer programming, ?: is a ternary operator that is part of the syntax for a basic conditional expression in
several programming languages. It is commonly referred to as the conditional operator.
It originally comes from BCPL, whose equivalent syntax for e1 ? e2 : e3 was e1 -> e2, e3 [1] . Languages derived
from BCPL tend to feature this operator.
Conditional assignment
?: is used as follows:
condition ? value if true : value if false
The condition is evaluated true or false as a Boolean expression. On the basis of the evaluation of the Boolean
condition, the entire expression returns value if true if condition is true, but value if false otherwise. Usually the two
sub-expressions value if true and value if false must have the same type, which determines the type of the whole
expression. The importance of this type-checking lies in the operator's most common use—in conditional assignment
statements. In this usage it appears as an expression on the right side of an assignment statement, as follows:
variable = condition ? value if true : value if false
The ?: operator is similar to the way conditional expressions (if-then-else constructs) work in functional
programming languages, like Scheme, ML, and Haskell, since if-then-else forms an expression instead of a statement
in those languages.
Usage
This ternary operator's most common usage is to make a terse simple conditional assignment statement. For example,
if we wish to implement some C code to change a shop's opening hours to 12 o'clock in weekends, and 9 o'clock on
weekdays, we may use
int opening_time = (day == WEEKEND) ? 12 : 9;
instead of the more verbose
int opening_time;
if (day == WEEKEND)
else
opening_time = 12;
opening_time = 9;
The two forms are nearly equivalent. Keep in mind that the ?: is an expression and if-then-else is a statement. Note
that neither value if true nor value if false expressions can be omitted from the ternary operator without an error
report upon parsing. This contrasts with if..else statements, where the else clause can be omitted.