JavaScript-Interpreter in Python

A JavaScript Interpreter coded in Python using Lark parsing and lexing library

Lark: https://lark-parser.readthedocs.io/en/stable/#
JavaScript grammar reference: https://tc39.es/ecma262/
JavaScript Error Reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors?retiredLocale=it

Restriction of the grammar adopted

Data types

• Number
• Boolean
• String
• Array

Arithmetic Operations (with type coercion simulation)

• addition (+)
• subtraction (-)
• multiplication (*)
• division (/)

Relational Operations (with type coercion simulation)

• equal (==)
• not equal (!=)
• strict equal (===)
• strict not equal (!==)
• greater than (>)
• less than (<)
• greater than or equal to (>=)
• less than or equal to (<=)

Assignment operations (with type coercion simulation)

• identifier += expression
• identifier -= expression
• identifier *= expression
• identifier /= expression
• identifier ++ (post-increment)
• ++ identifier (pre-increment)
• identifier -- (post-decrement)
• -- identifier (pre-decrement)

Logical operations (with type coercion simulation)

• not (!)
• or (||)
• and (&&)

Branching operations

• if
• if else
• ternary conditional operator (condition ? true : false)

Loop operations

• while

Input instruction

• keyboard input (prompt)

Output instruction:

• print in console (console.log)
• template literals

Function declaration

• function IDENTIFIER (list_of_parameters) { body }

Function call

• IDENTIFIER (list_of_arguments)

Array

• array declaration
• array access (with integer index and non integer index)
• array length (length)

Comments (ignored)

• single line comment (//)
• multi line comment (/* */)

Managed errors

• Lexical errors (typing error)

• Syntax errors

  • non-matched parenthesis
  • identifier already declared
  • assignment to constant variable
  • redeclaration with different declaration type
  • reserved word used as identifier
  • unexpected end of input
  • missing equal sign in constant declaration

• Semantic errors

  • reference error
  • type error

Tests (folder javascript_tests)

The following test scripts are available:

• test_1.js: test the arithmetic and the relational operations and the type coercion.
• test_2.js: test the function declaration and the function call and the array declaration, access and length.
• test_3.js: test the branching operations (if, if else) and the loop operations (while) and array declaration, access and length.
• test_4.js: generate a lexical errors.
• test_5.js: generate a syntax errors.
• test_6.js: generate a semantic error.
• test_7.js: generate a type error.

Instructions to run the interpreter

1. clone the repository or download the project
2. install python (our python version: 3.8)
3. install lark library: pip install lark (our version: 1.1.7)

Script execution

1. open a terminal
2. go to the project folder
3. run the script: python main.py -s (--script) <path_to_script>
4. the output will be printed in the terminal

Console execution

1. open a terminal
2. go to the project folder
3. run the script: python main.py -c (--console) or python main.py
4. write the script in the console
5. press enter
6. the output will be printed in the terminal

For both the execution modes, if you specify the flag -d (--debug) the debug mode will be activated and the Parse Tree will be printed in the terminal.

Executable file execution

You can run the executable file for your operating system (Windows or MacOS) by following the instructions in the pre-release v0.1.0-alpha.