javascript eval method

How can I watch variable values inside of javascript eval() method? And is it po开发者_如何学Gossible to "step into" and "step over" in eval method? For example, with a code like this:

eval("if (true) { var a = 10; a += 20; alert(a); }");

I am more interested in debugging in IE9, but I would like to hear general principle as well.

you can't inside the eval method. the code you give it is no code but a string. after the eval() then it becomes code and you can inspect it. This also depends on what tools you use to debug your script. In Chrome if you click Pause on exception, and execute your eval. it will break (because b is undefined) and jump in the evaluated code where you can step over, in and out. you can also use new Function("code goes here")() to evaluate code

if variable is not exist how could you know it's value? - noway. And... eval === evil.

It is possible. you have to use SourceMap with your source code. Take a look at the source map which has en extensive information about this technique.

Although not thoroughly documented or advised, you can watch local variables.

var reference;

function alerta(){
  alert(reference[0]);
}

// Desired to be watched must be called as function arguments
(function(a){
  reference = arguments;

  a = 'taco';
  alerta();
  a = 'updatedValue';
  alerta();

})()

http://jsbin.com/oragan