src/interpreter/index.ts

// https://en.wikipedia.org/wiki/Interpreter_pattern

namespace InterpreterPattern {
  interface IAbstractExpression {
    interpret(): number;
  }

  class Numeral implements IAbstractExpression {
    // Terminal Expression
    value: number;

    constructor(value: string) {
      this.value = parseInt(value);
    }

    interpret(): number {
      return this.value;
    }
  }

  class Add implements IAbstractExpression {
    // Non-Terminal Expression.
    left: IAbstractExpression;
    right: IAbstractExpression;

    constructor(left: IAbstractExpression, right: IAbstractExpression) {
      this.left = left;
      this.right = right;
    }

    interpret() {
      return this.left.interpret() + this.right.interpret();
    }
  }

  class Subtract implements IAbstractExpression {
    // Non-Terminal Expression.
    left: IAbstractExpression;
    right: IAbstractExpression;

    constructor(left: IAbstractExpression, right: IAbstractExpression) {
      this.left = left;
      this.right = right;
    }

    interpret() {
      return this.left.interpret() - this.right.interpret();
    }
  }

  // Client code
  const SENTENCE = "5 + 4 - 3 + 7 - 2";
  console.log(SENTENCE);
  const TOKENS = SENTENCE.split(" ");
  console.log(JSON.stringify(TOKENS));

  const AST: IAbstractExpression[] = []; // An array of AbstractExpressions
  AST.push(new Add(new Numeral(TOKENS[0]), new Numeral(TOKENS[2]))); // 5 + 4
  AST.push(new Subtract(AST[0], new Numeral(TOKENS[4]))); // ^ - 3
  AST.push(new Add(AST[1], new Numeral(TOKENS[6]))); // ^ + 7
  AST.push(new Subtract(AST[2], new Numeral(TOKENS[8]))); // ^ - 2

  const AST_ROOT = AST.pop();

  console.log((AST_ROOT as IAbstractExpression).interpret());
  console.dir(AST_ROOT, { depth: null });

  // Output
  /*
    5 + 4 - 3 + 7 - 2
    ["5","+","4","-","3","+","7","-","2"]
    11
    Subtract {
    left: Add {
        left: Subtract {
        left: Add { left: Numeral { value: 5 }, right: Numeral { value: 4 } },
        right: Numeral { value: 3 }
        },
        right: Numeral { value: 7 }
    },
    right: Numeral { value: 2 }
    }
  */
}