javascript中的范围以访问私有变量

Question

a = function (){
    var b = 10;
    var k = function(){
        console.log(b);
    }();
}();

The above code will print 10.

var k = function(){
    console.log(b);
}
var a = function (){
    var b = 10;
    k();
}();

This code will print undefined .

Is it possible to print 10 instead? Like binding the scope to k before calling it.

Answer 1

As @Derek answered, you can obviously pass an argument.

Aside from that, you can't transfer or change variable scope, but you can directly manipulate calling context.

As such, you can set b as the property of an object, and set that object as the calling context of whatever function you're calling.

var k = function(){
    console.log(this.b);
}
var a = function (){
    var obj = {b:10};
    k.call(obj);
}();

The .call() method invokes the k function, but sets the first argument you provide as the calling context of the called function.

In fact, you don't technically need to use an object. You could directly set the number as the value to use...

var k = function(){
    console.log(this);
}
var a = function (){
    var b = 10;
    k.call(b);
}();

In strict mode, you'll get the number, but in non-strict, you'll get the Number as its object wrapper.

Answer 2

Really? What have you tried?

var k = function(b){
    console.log(b);
}
var a = function (){
    var b = 10;
    k(b);  //Done.
}();

Why does it give you undefined in your second example is because b is not defined. You have to define it first before using the variable b .

var k = function(b){ //<--There, defined. Mission accomplished.

Answer 3

Access into the scope can be granted through eval .

var a = (function() {
    var b = 10;
    return {
        eval: function(s) {
            return eval(s);
        }
    };
})();

var k = function() {
    console.log(a.eval("b"));
};

k();
​