README.md

A Simple Translator from Infix minus expressions to JavaScript

Install dependencies and run the tests:

➜  hello-jison git:(master) cd ast
➜  ast git:(master) npm test

> [email protected] test
> cd ast && npx mocha test/test.mjs



  ✓ transpile(test1.calc, out1.js)
  ✓ transpile(test2.calc, out2.js)
  ✓ transpile(test3.calc, out3.js)

  3 passing (13ms)

test folder

In this folder we have the data folder with

• the input files test1.calc, test2.calc and test3.calc
• the expected javascript output files correct1.js, correct2.js and correct3.js
• the expected output files correct-out1.txt, correct-out2.txt and correct-out3.txt.

➜  ast git:(master) tree test
test
├── data
│   ├── correct-out1.txt
│   ├── correct-out2.txt
│   ├── correct-out3.txt
│   ├── correct1.js
│   ├── correct2.js
│   ├── correct3.js
│   ├── test1.calc
│   ├── test2.calc
│   └── test3.calc
├── test-description.mjs
└── test.mjs

1 directory, 11 files

The script calc2js.mjs is a simple translator from infix minus expressions to JavaScript:

➜  ast git:(master) ✗ ./calc2js.mjs --help
Usage: calc2js [options] <filename>

Arguments:
  filename                 file with the original code

Options:
  -V, --version            output the version number
  -o, --output <filename>  file in which to write the output
  -h, --help               display help for command
➜  ast git:(master) ✗ cat test/data/test1.calc 
4 - 2 - 1
➜  ast git:(master) ✗ ./calc2js.mjs test/data/test1.calc    
console.log(4 - 2 - 1);

minus-ast.jison

The file minus-ast.jison is a simple parser from infix minus expressions expr producing a espree compatible AST for the statement console.log(expr).

%{
const { buildRoot, buildBinaryExpression, buildLiteral } = require('./ast-build');
%}

%left '-'
%%
es: e { return buildRoot($1); }
;

e: 
    e '-' e  { $$ = buildBinaryExpression($1, '-', $3)  }
  | N        { $$ = buildLiteral($1); }
;

useminus.js

The script use_minus.js is a simple parser from infix minus expressions expr producing a espree compatible AST for the statement console.log(expr). That is, we
wrap the input expression with a console.log statement.

ast git:(master) ./use_minus.js "2-1-1" > ast.json

See the file ast.json for the output of the command above.

ast2js.js

The script ast2js.js uses escodegen.generate to traverse a espree compatible AST producing as output the corresponding JavaScript:

➜  ast git:(master) ✗ ./ast2js.js './ast.json'
console.log(2 - 1 - 1);

ast-build.js

buildLiteral

It is simple to build the tree for a literal:

function buildLiteral(value) {
  return {
    type: "Literal",
    value: Number(value),
    raw: value,
  };
}

buildBinaryExpression

To build the tree for a binary expression, we build the left and right children and then the binary expression itself:

function buildBinaryExpression(left, op, right) {
  return {
    type: "BinaryExpression",
    left: left,
    operator: op,
    right: right,
  };
}

buildRoot

The function buildRoot in ast-build.js wraps the JS expression of the AST in a console.log statement.:

function buildRoot(child) {
  return {
    type: "Program",
    body: [
      {
        type: "ExpressionStatement",
        expression: {
          type: "CallExpression",
          callee: {
            type: "MemberExpression",
            object: {
              type: "Identifier",
              name: "console",
            },
            property: {
              type: "Identifier",
              name: "log",
            },
            computed: false,
          },
          arguments: [child],
        },
      },
    ],
    sourceType: "script",
  };
}