Interpreter Design Pattern

Purpose: Behavioural Type: Class

The Interpreter Design Pattern specifies how to evaluate sentences in a string.

The idea is to have a class for each symbol. The syntax tree of a sentence in the language is an instance of the composite pattern and is used to evaluate (intrepret) the sentence for a client.


  • AbstractExpression (declares interpret method common to all nodes)
  • TerminalExpression (interpret terminal symbols in the grammar)
  • NonTerminalExpression (maintains instance variables of type AbstractExpression for nonterminal symbols in grammar)
  • Context (info global to interpreter)
  • Client (builds abstract syntax tree representing a particular sentence in the language that the grammar defines)


class Context { private _input; constructor(input) { this._input = input; } public set input(value) { this._input = value; } } interface Expression { interpret(context: Context): void; } class TerminalExpression implements Expression { interpret(context: Context): void { // process comma console.log('interpret terminal'); } } class NoneTerminalExpression implements Expression { interpret(context: Context): void { // process variable console.log('interpret nonterminal'); } } (function main() { const context: Context = new Context('1 bird can fly'); const list = []; list.push(new TerminalExpression()); list.push(new NoneTerminalExpression()); list.map((exp: Expression) => { exp.interpret(context); }); })();