Flanagan's "JavaScript - The Definitive Guide" gives the following on page 653:

var variables = ""
for (var name in this)
    variables += name + "\n";

tl;dr: Don't use eval!

There is no single solution for this. It is possible to access some global variables dynamically via window, but that doesn't work for variables local to a function. Global variables that do not become a property of window are variables defined with let and const, and classes.

There is almost always a better solution than using variable variables! Instead you should be looking at data structures and choose the right one for your problem.

If you have a fixed set of names, such as

// BAD - DON'T DO THIS!!!
var foo = 42;
var bar = 21;

var key = 'foo';
console.log(eval(key));

store those names/values as properties of an object and use bracket notation to look them up dynamically:

// GOOD
var obj = {
  foo: 42,
  bar: 21,
};

var key = 'foo';
console.log(obj[key]);

In ES2015+ it's even easier to do this for existing variables using concise property notation:

// GOOD
var foo = 42;
var bar = 21;
var obj = {foo, bar};

var key = 'foo';
console.log(obj[key]);

If you have "consecutively" numbered variables, such as

// BAD - DON'T DO THIS!!!
var foo1 = 'foo';
var foo2 = 'bar';
var foo3 = 'baz';

var index = 1;
console.log(eval('foo' + index));

then you should be using an array instead and simply use the index to access the corresponding value:

// GOOD
var foos = ['foo', 'bar', 'baz'];
var index = 1;
console.log(foos[index - 1]);