jscodeshift-comparision.md

I a previous post, we introduced JSFIX and briefly made an informal comparison with other codemod tools. We argued that JSFIX is a semantics-based codemod tool, which considers what your code does, as opposed to most other syntax-based codemod tools, which only consider how your code looks. In this post, we will examine this point further, by introducing JSFIX's semantic patch language.

A semantic patch language

We borrow the term semantic patch from Lawall et al. In Lawall's work, a semantic patch is a SmPL (Semantic patch language) program used by the Coccinelle program to migrate a Linux driver written in C from an older version to a newer version of the Linux kernel.

JSFIX has its own semantic patch language, which, analogous to Lawall's, serves to migrate JavaScript/TypeScript programs from an older to a newer version of some npm dependency.
Apart from the somewhat similar purposes, JSFIX and Coccinelle and their semantic patch languages are otherwise quite different.

Let's try to write a semantic patch for the Lodash breaking change introduced in the previous post. Recall that we want to transform

var l = require('lodash');
function f () {
    l.reduce([1, 2, 3], (t, n) => t + n, 0, this);
}

to

var l = require('lodash');
function f () {
    l.reduce([1, 2, 3], ((t, n) => t + n).bind(this), 0);
}

in other words, we want to take the fourth argument of calls to reduce and pass that argument to a bind method call we insert on the second argument.
A JSFIX semantic patch for performing this transformation reads:

$\texttt{call <lodash>.reduce [4,4]} \leadsto \texttt{callee(1, 2.bind(4), 3)}$.