为什么在回调中声明var不能定义参数？ （起重）

Question

$(function() {
    foo1("text");
    foo2("text");
}) 

function callback(func) {
    func();
}

function foo1(bar) {
    callback(function() {
        console.log(bar);       // "text"
        bar = "derp";
        console.log(bar);       // "derp"
    })
}

function foo2(bar) {
    callback(function() {
        console.log(bar);       // undefined
        var bar = "derp";
        console.log(bar);       // "derp"
    })
}

Why is that declaration of var bar = "derp" undefining the parameter, that is accessed beforehand?

Answer 1

Because this code

function foo2(bar) {
    callback(function() {    
       console.log(bar);       // undefined
       var bar = "derp";
       console.log(bar);       // "derp"
    })
}

is actually

function foo2(bar) {
    callback(function() {  
       var bar;
       console.log(bar);       // undefined
       bar = "derp";
       console.log(bar);       // "derp"
    })
}

due to the variable hoisting . So even if you created a global bar variable inside the foo1 call, a scoped variable bar is declared inside the inner scope of the foo2 function. That's why the first console.log returns undefined

See also Variable hoisting on SO