Elvis operator

In certain computer programming languages, the Elvis operator, often written ?:, is a binary operator that returns the evaluated first operand if that operand evaluates to a value likened to logically true (according to a language-dependent convention, in other words, a truthy value), and otherwise returns the evaluated second operand (in which case the first operand evaluated to a value likened to logically false, in other words, a falsy value). This is identical to a short-circuit or with "last value" semantics. The notation of the Elvis operator was inspired by the ternary conditional operator, ? :, since the Elvis operator expression A ?: B is approximately equivalent to the ternary conditional expression A ? A : B.

The name "Elvis operator" refers to the fact that when its common notation, ?:, is viewed sideways, it resembles an emoticon of Elvis Presley with his signature hairstyle.^[1]

A similar operator is the null coalescing operator, where the boolean truth(iness) check is replaced with a check for non-null instead. This is usually written ??, and can be seen in languages like C#^[2] or Dart.^[3]

In several languages, such as Common Lisp, Clojure, Lua, Object Pascal, Perl, Python, Ruby, and JavaScript, the logical disjunction operator (typically || or or) has the same behavior as the above: returning its first operand if it would evaluate to a truthy value, and otherwise evaluating and returning its second operand, which may be a truthy or falsy value. When the left-hand side is truthy, the right-hand side is not even evaluated; it is "short-circuited". This is different than the behavior in other languages such as C/C++, where the result of || will always be a (proper) boolean.

In a language that supports the Elvis operator, something like this:

x = f() ?: g()

will set x equal to the result of f() if that result is truthy, and to the result of g() otherwise.

It is equivalent to this example, using the conditional ternary operator:

x = f() ? f() : g()

except that it does not evaluate f() twice if it yields truthy. Note the possibility of arbitrary behaviour if f() is not a state-independent function that always returns the same result.

This code will result in a reference to an object that is guaranteed to not be null. Function f() returns an object reference instead of a boolean, and may return null, which is universally regarded as falsy:

x = f() ?: "default value"

• In GNU C and C++ (that is: in C and C++ with GCC extensions), the second operand of the ternary operator is optional.^[4] This has been the case since at least GCC 2.95.3 (March 2001), and seems to be the original Elvis operator.^[5]
• In Apache Groovy, the "Elvis operator" ?: is documented as a distinct operator;^[6] this feature was added in Groovy 1.5^[7] (December 2007). Groovy, unlike GNU C and PHP, does not simply allow the second operand of ternary ?: to be omitted; rather, binary ?: must be written as a single operator, with no whitespace in between.
• In PHP, it is possible to leave out the middle part of the ternary operator since PHP 5.3.^[8] (June 2009).
• The Fantom programming language has the ?: binary operator that compares its first operand with null.
• In Kotlin, the Elvis operator returns its left-hand side if it is not null, and its right-hand side otherwise.^[9] A common pattern is to use it with return, like this: val foo = bar() ?: return
• In Gosu, the ?: operator returns the right operand if the left is null as well.
• In C#, the null-conditional operator, ?. is referred to as the "Elvis operator",^[10] but it does not perform the same function. Instead, the null-coalescing operator ?? does.
• In ColdFusion and CFML, the Elvis operator was introduced using the ?: syntax.
• The Xtend programming language has an Elvis operator.^[11]
• In Google's Closure Templates, the Elvis operator is a null coalescing operator, equivalent to isNonnull($a) ? $a : $b.^[12]
• In Ballerina, the Elvis operator L ?: R returns the value of L if it's not nil. Otherwise, return the value of R.^[13]
• In JavaScript, the nullish coalescing (??) operator is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand.^[14]

• ?: or conditional operator, when used as a ternary operator
• Safe navigation operator, often ?.
• Spaceship operator <=>
• Option type