A pen question is the result of thinking that normally no one would change the parameter values inside a function. Just for discussion here, there are three ways to modify.

**1. Directly modify the parameters of the function declaration **

function f1(a) {
    alert(a);
    a = 1;//Modify parameter a
    alert(1 === a);
    alert(1 === arguments[0]);
}
f1(10);

Function f1 defines parameter a, when called, pass parameter 10, pop up 10 first, change a to 1, pop up true twice, a and arguments[0] are 1.

**2, via the arguments object inside the function **

function f2(a) {
    alert(a);
    arguments[0] = 1;//Modify the arguments
    alert(1 === a);
    alert(1 === arguments[0]);

}

The effect is the same as the function f1.

**3. The local variable declared inside the function has the same name as the parameter **

function f3(a) {
    alert(a);
    var a = 1;//Declare the local variable a and assign it a value of 1
    alert(1 === a);
    alert(arguments[0]);
}
f3(10);

The function f3 defines the parameter a. The function internally declares the local variable a and assigns a value of 1, but here the a is still the parameter a, which can be proved by the argument [0] that pops up at the end being changed to 1.

**4. If you just declare the local variable a and don’t assign a value, that’s different **

function f3(a) {
    var a;//Declaration only, no assignment
    alert(a);
    alert(arguments[0]);
}
f3(10);

Instead of undefined, all of the 10’s that pop up.