Question

I know that eval and setTimeout can both accept a string as the (1 st) parameter, and I know that I’d better not use this. I’m just curious why is there a difference:

!function() {
    var foo = 123;
    eval("alert(foo)");
}();

!function() {
    var foo = 123;
    setTimeout("alert(foo)", 0);
}();

the first would work, and the second will give an error: foo is not defined

How are they executed behind the scene?

Answer 1

See the reference of setTimeout on MDN.

String literals are evaluated in the global context, so local symbols in the context where setTimeout() was called will not be available when the string is evaluated as code.

In contrast, the string literal passed to eval() is executed in the context of the call to eval.