When comparing Flow vs PharoJS, the Slant community recommends PharoJS for most people. In the question“What are the best solutions to "The JavaScript Problem"?” PharoJS is ranked 20th while Flow is ranked 32nd. 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 Checks to see if you check for Nulls
Because getting those exceptions is just not fun and very pervasive.
Pro Versioned type definitions
Pro There is support in many code editors via the extension
For example, there is good support through the extension in Visual Studio Code, which is a good editor for TypeScript, which is a competitor to Flow.
Pro Babel extension for strip of type annotations
Thanks to the Babel extension for the output, there is minimally modified code that is understandable to the author.
Pro Statical analysing of JavaScript code
Statical analysing of JavaScript code without pre-making any changes to it. But supported annotation types by extending the syntax of the 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 Weak base type definitions even for popular JavaScript libraries
For example, there are definitions for Gulp, React.