ast-eval

Statically evaluate expressions in AST, also known as constants folding. Useful for precompilation tasks.

Use

npm install --save ast-eval

var esprima = require('esprima');
var gen = require('escodegen').generate;
var astEval = require('ast-eval');
 
var ast = esprima.parse('[1, 2 === "2", 3+4*10, [2] === 2]');
ast = astEval(ast);
 
gen(ast); //'[1, false, 43, false]'

API

preeval(Node, options) → Node

Evaluate expressions in a Node, return a new Node with optimized shorten expression nodes.

Features

• Fold expressions

  • Binary expressions: 1000 * 60 * 60 → 36e6
  • Logical expressions: {a:1} && {b:2} → true
  • Math expressions: Math.sin(Math.Pi / 2 ) → 1

• Fold arrays

  • Safe methods: [1,2,3,new Date].concat(4, [5], new Date) → [1,2,3,new Date,4,5, new Date]
  • Unsafe methods: [1,2,3].map(function(x){ return x*2}) → [2,4,6]
  • Static methods: Array.from([1, 2, 3], function(x){ return x*2; }) → [2,4,6]
  • Prototype methods: Array.prototype.slice.call([1,2,3], 1,2) → [2]

• Fold static globals (what’s that?)

• Decompute object access (optionally)

  • a['x'] = 1 → a.x = 1

• Fold strings

  • 'a b c'.split(' ') → ['a', 'b', 'c']

• Propagate constants

  • Simple flow analysis: var x = 1; x + 2; → 3;
  • Scope analysis
  • Method substitution: var slice = Array.prototype.slice; var x = [1,2,3]; var y = slice(x)'

• Fold loops

  • var x = []; for (var i = 0; i < 10; i++) {x[i] = 10*i;}

• Fold proxy functions

• Remove unused props

• Undead code

  • Empty isolated functions
  • Remove unused variables (after enabling constants)
  • Remove unused functions
  • Remove unused properties

• Fold clone-code

  • a.x×3 → var _a = a; _a.x

• Data-flow analysis

  • Precall functions
  • Substitute variables

• Provide exports

• Fold primitives

  • new Array([1,2,3,...])
  • [1,2,3,...]

• Rearrange things

  • Hoist functions (place after first use)
  • Fold variable declarations

References

• List of compiler optimizations — ideas of folding.
• Substack’s static-eval — evaluate static expressions.
• esmangle