{"id":468,"date":"2009-10-20T14:59:30","date_gmt":"2009-10-20T19:59:30","guid":{"rendered":"http:\/\/vgable.com\/blog\/?p=468"},"modified":"2009-10-20T14:59:32","modified_gmt":"2009-10-20T19:59:32","slug":"javascript-nailed","status":"publish","type":"post","link":"https:\/\/vgable.com\/blog\/2009\/10\/20\/javascript-nailed\/","title":{"rendered":"JavaScript Nailed ||"},"content":{"rendered":"

One thing about JavaScript I really like is that its ||<\/code>, the Logical Or<\/code> operator, is really a more general ‘Eval Until True<\/code>‘ operation. (If you have a better name for this operation, please leave a comment!)<\/strong> It’s the same kind of or<\/code> operator used in Lisp<\/a>. And I believe it’s the best choice for a language to use.<\/p>\n

In C\/C++, a || b<\/code> is equivalent to,<\/p>\n

\n  if a evaluates to a non-zero value:\n    return true;\n  if b evaluates to a non-zero value:\n    return true;\n  otherwise:\n    return false;\n<\/pre>\n
Note that if a<\/code> can be converted to true<\/code>, then b<\/code> is not<\/em> evaluated. Importantly, in C\/C++ ||<\/code> always returns a bool<\/code><\/strong>.<\/p>\n
But the JavaScript ||<\/code> returns the value<\/em> of the first variable that can be converted to true<\/code><\/strong>, or the last variable if both variables can’t be interpreted as true<\/code>,<\/p>\n
\n  if a evaluates to a non-zero value:\n    return a;\n  otherwise:\n    return b;\n<\/pre>\n
Concise<\/h3>\n
JavaScript’s ||<\/code> is some sweet syntactic sugar.<\/p>\n
We can write,<\/p>\n
return playerName || \"Player 1\";<\/pre>\n
instead of,<\/p>\n
return playerName ? playerName : \"Player 1\";<\/pre>\n
And simplify assert<\/code>-like code in a perl-esq way,<\/p>\n
x || throw \"x was unexpectedly null!\";<\/pre>\n
It’s interesting that a more concise definition of ||<\/code> allows more concise code, even though intuitively we’d expect a more complex ||<\/code> to “do more work for us”.<\/p>\n
General<\/h3>\n
Defining ||<\/code> to return values, not true<\/code>\/false<\/code>, is much more useful for functional programming.<\/p>\n
The short-circuit-evaluation is powerful enough to replace if<\/code>-statements. For example, the familiar factorial function,<\/p>\n
function factorial(n){\n\tif(n == 0) return 1;\n\treturn n*factorial(n-1);\n}<\/pre>\n
can be written in JavaScript using &&<\/code> and ||<\/code> expressions,<\/p>\n
function factorial2(n){ return n * (n && factorial2(n-1)) || 1;}<\/pre>\n
Yes, I know this isn’t the clearest way to write a factorial, and it would still be an expression if it used ?:<\/code>, but hopefully this gives you a sense of what short-circuiting operations can do.<\/p>\n
Unlike ?:<\/code>, the two-argument ||<\/code> intuitively generalizes to n<\/em> arguments, equivalent to a1 || a2 || ... || an<\/em><\/code>. This makes it even more useful for dealing with abstractions.<\/p>\n
Logical operators that return values, instead of simply booleans, are more expressive and powerful, although at first they may not seem useful — especially coming from a language without them.<\/p>\n","protected":false},"excerpt":{"rendered":"
One thing about JavaScript I really like is that its ||, the Logical Or operator, is really a more general ‘Eval Until True‘ operation. (If you have a better name for this operation, please leave a comment!) It’s the same kind of or operator used in Lisp. And I believe it’s the best choice for […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7,12,4,8],"tags":[613,326,394,512,417],"class_list":["post-468","post","type-post","status-publish","format-standard","hentry","category-c","category-design","category-programming","category-usability","tag-c","tag-functional-programming","tag-javascript","tag-look-at-my-stupid-factorial-function","tag-programming-language-design"],"_links":{"self":[{"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/posts\/468","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/comments?post=468"}],"version-history":[{"count":1,"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/posts\/468\/revisions"}],"predecessor-version":[{"id":469,"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/posts\/468\/revisions\/469"}],"wp:attachment":[{"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/media?parent=468"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/categories?post=468"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/vgable.com\/blog\/wp-json\/wp\/v2\/tags?post=468"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}