I've wanted to have , that will make it easier to create for my Open Source project . The idea for the language came after I've created a paid gig for one person, let's call him Ken, that needed this type of game, where the user interacted with the terminal and was asked a bunch of questions and it was like an adventure game, related to . The code I've written, that Ken needed, was data-driven by a JSON file. It was working nicely, Ken could easily change the JSON and have the game changed however he wanted. I've asked if I could share the code since it was a very cool project and Ken agreed that I can do that two months after he publish the game. But after a while, I've realized that I can have something much better. My own that will make it . A person with a bit of programming knowledge like Ken, could easily edit the game, because the language will be much simpler than complex JavaScript code that is needed for something like this. And even if I would be asked to create a game like the one for Ken, it would be much easier and faster for me. This is how has started. my own programming language text-based adventure games jQuery Terminal Crypo DSL language, simpler to create text-based adventure games Gaiman programming language I used before, so it was my obvious choice for the . First I've started with , modified it, and then added and .  When I've had this first proof of concept that was generating , I was so excited that I've had to write an article and share how simple it is to create . PEG.js parser generator an arithmetic example if statement boolean expressions output JavaScript code your own programming language in JavaScript In the end, there is a simple demo playground, and if you want something more cool look at the Gaiman website, link on . GitHub So to the point, let's dive in. What is a Compiler? is a program that translates code from one programming language to another language. For instance, the C compiler translates a program written in C language into machine code (binary that can be interpreted by the computer). But there are also compilers that translate one human-readable language into different readable language. For instance, ClojureScript is compiled into JavaScript. This process is often called transpiling and program that does this is often called transpiler. A Compiler What is a Parser? is a program that can be part of the compiler or . It takes input code as a sequence of characters and produces (Abstract Syntax Tree), which can be used by code generator (part of the compiler) to generate the output code or by the evaluator (part of interpreter) to execute it. A Parser interpreter AST What is AST? AST is an acronym for Abstract Syntax Tree. It's the way to represent code in a format that tools can understand usually in form of . We will use AST in the format of an , which is a JavaScript parser that outputs AST. tree data structure Esprima What is a Parser Generator? Parser generator as the name suggests is a program that generates the source code of a parser for you based on grammar (language specification). Written in a specific syntax. In this article, we will use PEG.js parser generator that generates a JavaScript code that will parse the code for your language and output AST. A Parser generator is also a compiler, so you can call it compiler compiler. A Compiler that can output a compiler for your language. JavaScript Code generation What's cool about Esprima syntax is that there are tools that generate code based on their AST. An example is which takes Esprima AST as input and outputs JavaScript code. You can think that you can use just strings to generate code, but this solution will not scale. In this tutorial, I show only a single if statement but you will run into a lot of problems if you will have more complex code. escodegen PEG.js parser for simple programming language PEG.js is a compiler for written in JavaScript. It takes simpler PEG language that uses inline JavaScript code and output a parser. Parsing expression Grammars Below I will show you how to create a simple parser PEG.js grammar for if statement that will output AST, which then later will be transformed into JavaScipt code. The syntax of PEG.js is not very complicated, it consists of the name of the rule, then the matching and optional block of JavaScript that is executed and returned from the rule. Here is a simple arithmetic example provided by PEG.js documentation: }
} additive multiplicative { function makeInteger(o) { return parseInt(o.join(""), 10 ); start = additive = left :multiplicative "+" right :additive { return left + right ; } / multiplicative = left : primary "*" right :multiplicative { return left * right ; } / primary primary = integer / "(" additive:additive ")" { return additive; } integer "integer" = digits:[ 0 -9 ] + { return makeInteger(digits); } The output parser from this grammar can parse and evaluate simple arithmetic expressions for example that evaluates to . You can test this parser at . Note that it doesn't handle spaces between tokens (to simplify the code), with a parser you need to handle this explicitly. 10+2*3 16 PEG.js Online Tool But what we need is not to interpret the code and return a single value but return Esprima AST. To see how Esprima AST looks like you can check select Esprima as output and type some JavaScript. AST Explorer Here is an example of simple code like this: } if (foo == "bar" ) { 10 + 10 10 * 20 The output in JSON format looks like this: { ]
        }, ]
        }, ]
      }, { ]
              }, ]
              }, ]
            }, ]
          },
          { ]
              }, ]
              }, ]
            }, ]
          }
        ], ]
      }, ]
    }
  ], ]
} { "type" : "Program" , "body" : [ "type" : "IfStatement" , "test" : { "type" : "BinaryExpression" , "operator" : "==" , "left" : { "type" : "Identifier" , "name" : "foo" , "range" : [ 4 , 7 "right" : { "type" : "Literal" , "value" : "bar" , "raw" : "\"bar\"" , "range" : [ 11 , 16 "range" : [ 4 , 16 "consequent" : { "type" : "BlockStatement" , "body" : [ "type" : "ExpressionStatement" , "expression" : { "type" : "BinaryExpression" , "operator" : "+" , "left" : { "type" : "Literal" , "value" : 10 , "raw" : "10" , "range" : [ 23 , 25 "right" : { "type" : "Literal" , "value" : 10 , "raw" : "10" , "range" : [ 28 , 30 "range" : [ 23 , 30 "range" : [ 23 , 30 "type" : "ExpressionStatement" , "expression" : { "type" : "BinaryExpression" , "operator" : "*" , "left" : { "type" : "Literal" , "value" : 10 , "raw" : "10" , "range" : [ 34 , 36 "right" : { "type" : "Literal" , "value" : 20 , "raw" : "20" , "range" : [ 39 , 41 "range" : [ 34 , 41 "range" : [ 34 , 41 "range" : [ 18 , 43 "alternate" : null , "range" : [ 0 , 43 "sourceType" : "module" , "range" : [ 0 , 43 You don't need to care about "range"  and "raw". They are part of the parser output. Let's split the JSON down into its part: If statement The if statement needs to be in the format: }, }, } { "type" : "IfStatement" , "test" : { "consequent" : { "alternate" : null Where "test" and "consequent are any expressions: if statement condition The condition can be any expression but here we will have a binary expression that compare two things: } { "type" : "BinaryExpression" , "operator" : "==" , "left" : {}, "right" : {} Variables Variables usage looks like this: } { "type" : "Identifier" , "name" : "foo" Literal string A literal string that is used in our code looks like this: } { "type" : "Literal" , "value" : "bar" Block with curly braces The block inside if is created like this: } { "type" : "BlockStatement" , "body" : [ ] Whole program And the whole program is created like this: } { "type" : "Program" , "body" : [ ] PEG Parser for your own language that compiles to JavaScript For our demo language we will create code that looks similar to ruby: end if foo == "bar" then 10 + 10 10 * 20 and we will create AST, which then will create JavaScript code. Peg grammar for if looks like this: return { }, };
} if = "if" _ expression:(comparison / expression) _ "then" body:(statements / _) _ "end" { "type" : "IfStatement" , "test" : expression, "consequent" : { "type" : "BlockStatement" , "body" : body "alternate" : null we have "if" token, then an expression that is comparison or expression and body is statements or white space. _ are optional whitespaces that are ignored. _ = [ \t\n\r]* The comparison looks like this: };
} comparison = _ left :expression _ "==" _ right :expression _ { return { "type" : "BinaryExpression" , "operator" : "==" , "left" : left , "right" : right The expression looks like this: expression = expression :(variable / literal) { return expression ; } Variable is created from three rules: return { }
} variable = !keywords variable:name { "type" : "Identifier" , "name" : variable keywords = "if" / "then" / "end" name = [A-Z_$a-z][A-Z_a-z0 -9 ]* { return text(); } Now let's look at statements: } return  { } statements = _ head:( if / expression_statement) _ tail:( ! "end" _ ( if / expression_statement))* { return [head].concat( tail.map ( function ( element ) { return element[ 2 ] ; })) ; expression_statement = expression :expression { "type" : "ExpressionStatement" , "expression" : expression } ; And the last thing is literals: } } literal = value:( string / Integer) { return { "type" : "Literal" , "value" : value } ; string = "\"" ([^ "] / " \\\\\ "" )* "\"" { return JSON.parse( text ()) ; Integer "integer" = _ [ 0 -9 ]+ { return parseInt( text (), 10 ) ; } And that is the whole parser, that generates AST. After we have Esprima AST all we have to do, is to generate the code with escodegen. The code that generates the AST and creates JavaScript code looks like this: const ast = parser.parse(code); const js_code = escodegen.generate(ast); the parser variable is the name that you give when you generate the parser using PEG.js. And here is a simple demo that I was using to write the parser, you can play with the grammar and generate different syntax for your own programming language that compiles to JavaScript. . Parser Generator Demo This simple application save your code in LocalStorage, If it compile without errors, on each change. So you can safely use it to create your own language. But I don't guarantee that you will not lose your work, so you may use something that is more robust. The original PEG.js project is not maintained anymore, but there is a new fork, that is maintained and it's backward compatible with PEG.js so it will be easy to switch. NOTE: Peggy Conclusion In this article we used parser generator to create . As you can see starting a project like this is not that hard. The techniques explained in this article should allow you to create any programming language that compiles to JavaScript on your own. This can be a way to create a PoC of a language that you want to design. As far as I know, this is the fastest way to have something working. But you can use your language as is and create your own DLS (Domain Specific Language), write code in that language, and make JavaScript do the hard work. simple custom language to JavaScript compiler

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