评估和功能范围

Question

Something seems wrong with the following code. It declares a function with eval , calls it — until then, everything is fine — and, calls it again but, though a function and thus gets an error.

var fn = function() {
    return isTwo(2);
};
var check = function() {
    eval("var isTwo = function(value) { return value == 2 }")
    console.log(isTwo(2)); // returns true
    console.log(fn()); // returns isTwo is not defined
}
check();

Unwrapping the check function made things works, but seems wrong. Why using eval inside of a function should change its behavior?

var fn = function() {
    return isTwo(2);
};

eval("var isTwo = function(value) { return value == 2 }")
console.log(isTwo(2)); // returns true
console.log(fn()); // returns true

Answer 1

Because eval acts as if you had replaced the line with the code to be evaluated. Therefore, var isTwo = function(value) { return value == 2 } defines a local variable, and it can't be accessed by your other function. The reason it works in the outer block is because it is then a global variable, and can be accessed by your other function.