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

/**
 * 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
 */
module.exports = function compileInlineExpression (expression, math, scope) {
  // find an undefined symbol
  var symbol = expression.filter(function (node) {
    return (node && node.isSymbolNode) &&
        !(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);
  }
};