side-effects-safe

Provides lists of side-effects free functions and a helper to check if an abstract syntax tree expression node can be dropped if its value is not used.

Statically deciding if a piece of code changes program state is not trivial – Rice's theorem suggests that the problem is in general undecidable. However, side-effects safety (with false negatives) may suffice for optimization purposes.

Pure function lists

Three lists of functions are provided:

• pureFuncs

  Calls to functions from this list should be safe to drop if the return value is not used (unless you do some nasty overriding).

• pureFuncsWithUnusualException

  These functions might throw a (commonly unhandled) exception in rare circumstances. However, it should be "almost correct" to consider them pure (for the purpose of optimizing a web application). For instance, isFinite() and isNaN() may throw a TypeError if given a Symbol. ¹

• pureFuncsWithTypicalException

  Functions that do not have side-effects, but do throw exceptions that are commonly handled. An example is JSON.parse() throwing SyntaxError for malformed JSON.

_{1. See [ToNumber()](https://tc39.github.io/ecma262/#sec-tonumber). Technically, some expressions may also throw a `TypeError`, e.g. `+Symbol.for(0)`.}

Usage with Uglify

Pass one of the provided lists as the pure_funcs compressor option:

export PURE_FUNCS=`nodejs -p "require('side-effects-safe').pureFuncs"`
uglifyjs -c pure_funcs="$PURE_FUNCS" bundle.js

Usage through Webpack

The same within a webpack.config.js:

const pureFuncs = require('side-effects-safe').pureFuncs;
const webpack = require('webpack');
 
module.exports = {
    ...
    plugins: [
        new webpack.optimize.UglifyJsPlugin({compress: {pure_funcs: pureFuncs}})
    ]
};

AST expression tester

Babylon 6

A single pure(node, opts) function meant to be used by Babel transforms is currently available. The function checks if an AST expression node could be dropped if we knew that its value is not going to be used:

import * as t from 'babel-types';
import {pureBabylon as pure} from 'side-effects-safe';
 
pure(t.binaryExpression('*', t.identifier('a'), t.identifier('b')));  // true
pure(t.updateExpression('++', t.identifier('a')));  // false

Options

You may pass a regular expression matching accessor chains that can be assumed pure:

let ex = t.memberExpression(t.identifier('a'), t.identifier('b'));  // a.b
pure(ex);  // false
pure(ex, {pureMembers: /^a\.b$/});  // true

Similarly for callee expressions:

let ex = t.callExpression(t.identifier('f'), []);  // f()
pure(ex);  // false
pure(ex, {pureCallees: /^f$/});  // true

Regexes are matched against a string with dots, irrespective of the property style used in code (for example a[3].b['c'] is normalized to a.3.b.c and a[b] never matches).

Example

Here's a sketch of a transform to remove no-op expression statements:

import {pureBabylon as pure} from 'side-effects-safe';
 
export default () => ({
    visitor: {
        ExpressionStatement(path) {
            if (pure(path.node.expression)) {
                path.remove();
            }
        }
    }
});

This is implemented in babel-remove-pure-exps. Take a look at babel-remove-props for another usage example.

Installation

npm install side-effects-safe

The pure function lists were produced by skimming a draft of the ES7 standard from February 2016. If you note something inaccurate or outdated please open an issue.