Flow is a static type checker, one that ensures that when a program is run that its variables, functions and other elements of code will adhere to their original specifications. It can check to see if a value assigned to a variable is the correct type, for instance, a number rather than a string of characters. It can also check to see if a function is passed the correct number of inputs.
Such error checks could prevent program failures, and help secure the program against malicious misuse. Type checking can also aid in maintaining the program's code base.
Running Flow does not require the programmer to compile the program, or otherwise prepare the program for inspection. Instead, it works as a background process. Flow can check standard primitive types such as numbers, strings, and Boolean values as well as structured types such as functions, objects, and arrays.
Flow is initially available as a plug-in for both the Emacs and Vim code editors, and versions for other editors may be released in the future. It also offers a API (application programming interface) so it can be used by other code analysis tools.