mathjs/lib/expression/transform/utils/compileInlineExpression.js

function factory (type, config, load, typed) {
  /**
   * Compile an inline expression like "x > 0"
   * @param {Node} expression
   * @param {Object} math
   * @param {Object} scope
   * @return {function} Returns a function with one argument which fills in the
   *                    undefined variable (like "x") and evaluates the expression
   */
  return function compileInlineExpression(expression, math, scope) {
    // find an undefined symbol
    var symbol = expression.filter(function (node) {
      return type.isSymbolNode(node) &&
          !(node.name in math) &&
          !(node.name in scope);
    })[0];

    if (!symbol) {
      throw new Error('No undefined variable found in inline expression "' + expression + '"');
    }

    // create a test function for this equation
    var name = symbol.name; // variable name
    var subScope = Object.create(scope);
    var eq = expression.compile();
    return function inlineExpression(x) {
      subScope[name] = x;
      return eq.eval(subScope);
    }
  };
}

exports.factory = factory;