computing Computing is any goal-oriented activity requiring, benefiting from, or creating computer, computing machinery. It includes the study and experimentation of algorithmic processes, and the development of both computer hardware, hardware and softw ...

, tree shaking is a

dead code elimination In compiler theory, dead-code elimination (DCE, dead-code removal, dead-code stripping, or dead-code strip) is a compiler optimization to remove dead code (code that does not affect the program results). Removing such code has several benefits: i ...

technique that is applied when optimizing code. Often contrasted with traditional single-library dead code elimination techniques common to minifiers, tree shaking eliminates unused functions from across the bundle by starting at the entry point and only including functions that may be executed. It is succinctly described as "live code inclusion".

History

Dead code elimination in dynamic languages is a much harder problem than in static languages. The idea of a "treeshaker" originated in

LISP Lisp (historically LISP, an abbreviation of "list processing") is a family of programming languages with a long history and a distinctive, fully parenthesized Polish notation#Explanation, prefix notation. Originally specified in the late 1950s, ...

in the 1990s. The idea is that all possible execution flows of a program can be represented as a tree of function calls, so that functions that are never called can be eliminated. The algorithm was applied to

JavaScript JavaScript (), often abbreviated as JS, is a programming language and core technology of the World Wide Web, alongside HTML and CSS. Ninety-nine percent of websites use JavaScript on the client side for webpage behavior. Web browsers have ...

in Google Closure Tools and then to Dart in the dart2js compiler also written by

Google Google LLC (, ) is an American multinational corporation and technology company focusing on online advertising, search engine technology, cloud computing, computer software, quantum computing, e-commerce, consumer electronics, and artificial ...

, presented by Bob Nystrom in 2012 and described by the book ''Dart in Action'' by author Chris Buckett in 2013: The next wave of popularity of the term is attributed to Rich Harris's Rollup projectHow To Clean Up Your JavaScript Build With Tree Shaking
/ref> developed in 2015.

Relation to ECMAScript 6 modules

The popularity of tree shaking in JavaScript is based on the fact that in contrast to CommonJS modules, ECMAScript 6 module loading is static and thus the whole dependency tree can be deduced by statically parsing the syntax tree. Thus tree shaking becomes an easy problem. However, tree shaking does not only apply at the import/export level: it can also work at the statement level, depending on the implementation.

References

{{reflist Compiler optimizations