When comparing Opa vs PharoJS, the Slant community recommends PharoJS for most people. In the question“What are the best languages that compile to JavaScript? ” PharoJS is ranked 13th while Opa is ranked 30th. The most important reason people chose PharoJS is:
For people who enjoy programming in Smalltalk, Pharo allows developers to use Smalltalk for web development as well, since it transpiles Smalltalk code to JavaScript.
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Pros
Pro Type Inferred
Opa uses type inference to decide what code runs on the client or the server, so you can organize code how you like, but still be sure that it runs where it needs to.
Pro Familiar Javascript Syntax
Opa's syntax is very similar to JavaScript, making it an easy choice for developers who are used to developing in JavaScript.
Pro Client and Server in the same language
Pro Program the web in Smalltalk
For people who enjoy programming in Smalltalk, Pharo allows developers to use Smalltalk for web development as well, since it transpiles Smalltalk code to JavaScript.
Pro Support for Phonegap to go directly to iOS/Android mobile App
Pro Compiles to very efficient Javascript, with almost perfect Smalltalk semantics
Pro Great IDE support
Pro Seamless integration with JavaScript libraries
Can directly call to / be called from JavaScript and can use foreign JavaScript objects.
Will soon have ability to generate and use AMD modules.
Cons
Con Not popular
Opa is not as popular as other languages which compile to JavaScript. Making it harder to find learning resources or even answers if you get stuck somewhere.