All about the with Statement in JavaScript: Removing with Statements in JavaScript Applications

摘要

The with statement in JavaScript makes static analysis of JavaScript applications difficult by introducing a new scope at run time and thus invalidating lexical scoping. Therefore, many static approaches to JavaScript program analysis and the strict mode of ECMAScript 5 simply disallow the with statement. To justify exclusion of the with statement, we should better understand the actual usage patterns of the with statement. In this paper, we present the usage patterns of the with statement in real-world JavaScript applications currently used in the 898 most popular web sites. We investigate whether we can rewrite the with statements in each pattern to other statements not using the with statement. We show that we can rewrite all the static occurrences of the with statement that do not have any dynamic code generating functions. Even though the rewriting process is not applicable to any dynamically generated with statements, our results are still promising. Because all the static approaches that disallow the with statement also disallow dynamic code generation, such static approaches can allow the with statement using our rewriting process. We formally present our rewriting strategy, provide its implementation, and show its faithfulness using extensive testing. We believe that removing with statements will simplify JavaScript program analysis designs without considering dynamic scope introduction while imposing fewer syntactic restrictions.