Specify scope for eval() in JavaScript?

Question

is there any way I can execute eval() on a specific scope (but NOT global)?

for example, the following code doesn\'t work (a is undefined on the sec

Answer 1

Here is a 20-line or so JS class that implements an extensible context using eval in a lexical scope:

// Scope class
//   aScope.eval(str) -- eval a string within the scope
//   aScope.newNames(name...) - adds vars to the scope
function Scope() {
  "use strict";
  this.names = [];
  this.eval = function(s) {
    return eval(s);
  };
}

Scope.prototype.newNames = function() {
  "use strict";
  var names = [].slice.call(arguments);
  var newNames = names.filter((x)=> !this.names.includes(x));

  if (newNames.length) {
    var i, len;
    var totalNames = newNames.concat(this.names);
    var code = "(function() {\n";

    for (i = 0, len = newNames.length; i < len; i++) {
      code += 'var ' + newNames[i] + ' = null;\n';
    }
    code += 'return function(str) {return eval(str)};\n})()';
    this.eval = this.eval(code);
    this.names = totalNames;
  }
}


// LOGGING FOR EXAMPLE RUN
function log(s, eval, expr) {
	s = '' + String(s);
  if (expr) {
    s += ':\n' + expr + '   -->   ';
  }
  s += '';
  if (expr) {
    try {
      s += '' + JSON.stringify(eval(expr)) + '';
    } catch (err) {
      s += '' + err.message + '';
    }
  }
  document.body.innerHTML += s + '\n\n';
}
document.body.innerHTML = '';


// EXAMPLE RUN
var scope = new Scope();
log("Evaluating a var statement doesn't change the scope but newNames does (should return undefined)", scope.eval, 'var x = 4')
log("X in the scope object should raise 'x not defined' error", scope.eval, 'x');
log("X in the global scope should raise 'x not defined' error", eval, 'x');
log("Adding X and Y to the scope object");
scope.newNames('x', 'y');
log("Assigning x and y", scope.eval, 'x = 3; y = 4');
log("X in the global scope should still raise 'x not defined' error", eval, 'x');
log("X + Y in the scope object should be 7", scope.eval, 'x + y');
log("X + Y in the global scope should raise 'x not defined' error", eval, 'x + y');

.remark {
  font-style: italic;
}

.result, .error {
  font-weight: bold;
}

.error {
  color: red;
}