swahili-lang/bin/interpreter/symbolTable.js

A new programming language with semantics borrowed from the Swahili language to help teach programming concepts to Swahili speaking students.

/**
 * map to hold all variable names and values in the current scope
 * and a pointer to its parent (if any)
 */
class SymbolTable {
  constructor(parent = null) {
    this.symbols = {};
    this.constants = {};
    this.parent = parent;
  }

  /**
   * accesses and returns the value of a variable
   * @param {String} name variable name to be accessed
   * @param {Boolean} shallow checks parent scope for variable if false
   */
  get(name, shallow = false) {
    const value = this.symbols[name] || null;
    if (!shallow && value === null && this.parent) return this.parent.get(name);

    return value;
  }

  /**
   * checks if given variable is a constant (in current or any containing scope)
   * @param {String} name name of the variable being searched for
   * @returns {Boolean}
   */
  hasConstant(name) {
    if (this.constants[name]) return true;
    if (this.parent) return this.parent.hasConstant(name);

    return false;
  }

  /**
   * assigns a value to a variable (or overrides existing value)
   * @param {String} name variable name to be assigned
   * @param {*} value value to be assigned to the variable
   * @param {Boolean} deep indicates whether to store variable in first pre-existing scope
   * @returns {Boolean} indicating whether value was updated
   */
  set(name, value, deep = false) {
    if (this.hasConstant(name)) return false;

    if (!deep) {
      this.symbols[name] = value;
    } else {
      // Traverse up the scope chain and assign the variable
      // in the first scope where it already existed
      if (!this.symbols[name] && this.parent) {
        this.parent.set(name, value, deep);
      } else {
        this.symbols[name] = value;
      }
    }

    return true;
  }

  /**
   * assigns a value to a variable as a constant
   * @param {String} name variable name to be assigned
   * @param {*} value value to be assigned to the variable
   */
  setConstant(name, value) {
    this.symbols[name] = value;
    this.constants[name] = true;
  }

  /**
   * deletes reference to variable name and value
   * @param {String} name variable name to be deleted
   */
  remove(name) {
    delete this.symbols[name];
  }
}

module.exports = SymbolTable;