JavaScript Interpreter Design Pattern

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Some applications are so complex that they require advanced configuration. You could offer a basic scripting language which allows the end-user to manipulate your application through simple instructions. The Interpreter pattern solves this particular problem – that of creating a simple scripting language.

Certain types of problems lend themselves to be characterized by a language. This language describes the problem domain which should be well-understood and well-defined. To implement this you need to map the language to a grammar. Grammars are usually hierarchical tree-like structures that step through multiple levels and then end up with terminal nodes (also called literals).

Problems like this, expressed as a grammar, can be implemented using the Interpreter design pattern.

Today, if you really need this type of control in JavaScript it is probably easier to use a code generator like ANTLR which will allow you to build your own command interpreters based on a grammar that you provide.

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The objects participating in this pattern are:

• Client -- In example code: the run() program.
  • builds (or is given) a syntax tree representing the grammar
  • establishes the initial context
  • invokes the interpret operations
• Context -- In example code: Context
  • contains state information to the interpreter
• TerminalExpression -- In example code: Expression
  • implements an interpret operation associated with terminal symbols in the grammar
  • one instance for each terminal expression in the sentence
• NonTerminalExpression -- In example code: not used
  • implements an interpret operation associated for non-terminal symbols in the grammar

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The objective of this example is to build an interpreter which translates roman numerals to decimal numbers: for example, XXXVI = 36.

The Context object maintains the input (the roman numeral) and the resulting output as it is being parsed and interpreted. The Expression object represents the nodes in the grammar tree; it supports the interpret method.

When running the program, a simple grammar tree is being built which then processes a roman numeral and translates it into a numeric.

var Context = function (input) {
    this.input = input;
    this.output = 0;
}

Context.prototype = {
    startsWith: function (str) {
        return this.input.substr(0, str.length) === str;
    }
}

var Expression = function (name, one, four, five, nine, multiplier) {
    this.name = name;
    this.one = one;
    this.four = four;
    this.five = five;
    this.nine = nine;
    this.multiplier = multiplier;
}

Expression.prototype = {
    interpret: function (context) {
        if (context.input.length == 0) {
            return;
        }
        else if (context.startsWith(this.nine)) {
            context.output += (9 * this.multiplier);
            context.input = context.input.substr(2);
        }
        else if (context.startsWith(this.four)) {
            context.output += (4 * this.multiplier);
            context.input = context.input.substr(2);
        }
        else if (context.startsWith(this.five)) {
            context.output += (5 * this.multiplier);
            context.input = context.input.substr(1);
        }
        while (context.startsWith(this.one)) {
            context.output += (1 * this.multiplier);
            context.input = context.input.substr(1);
        }
    }
}

function run() {
    var roman = "MCMXXVIII"
    var context = new Context(roman);
    var tree = [];

    tree.push(new Expression("thousand", "M", " ", " ", " ", 1000));
    tree.push(new Expression("hundred", "C", "CD", "D", "CM", 100));
    tree.push(new Expression("ten", "X", "XL", "L", "XC", 10));
    tree.push(new Expression("one", "I", "IV", "V", "IX", 1));

    for (var i = 0, len = tree.length; i < len; i++) {
        tree[i].interpret(context);
    }

    console.log(roman + " = " + context.output);
}

Run

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