Can eval() optimize?

Question

It is the ability of eval() to change local variables that is so problematic to JavaScript optimizers.

I read the book javascript Definitive Guide.

eval() can optimize ?? I don't understand.

Answer 1

It's not that eval optimizes, it's that JavaScript engines looking to optimize get prevented from doing so by eval, since it accepts a string and they cannot do static analysis on the code it may execute.

Consider this function:

function foo(str) {
    var a = getAReallyBigArray();
    doSomethingWith(a);
    document.getElementById("foo").addEventListener('click', function() {
        alert(str);
    }, false);
}

According to the specification, the event handler function has a reference to the a variable (through the lexical environment object for the execution context) and so the array is kept in memory for as long as that event handler exists. But a JavaScript engine can analyze the event handler and determine that it definitely doesn't reference a, and so optimize the contents of the variable binding object and let the array get garbage collected.

But throw an eval in there:

function foo(str) {
    var a = getAReallyBigArray();
    doSomethingWith(a);
    document.getElementById("foo").addEventListener('click', function() {
        eval(str);
    }, false);
}

Now, it's impossible for the JavaScript engine to optimize the contents of the lexical environment object, so it has to keep the big array in memory, in case str has code in it that accesses it.

That's just one specific example. The fundamental point is that eval throws a great big spanner in the works for the JavaScript engine's optimizer, effectively making the engine turn off its optimizer for the code in which eval appears.