你能动态地将局部变量添加到函数中吗？

Question

I'm using objects to namespace my JavaScript code. These objects usually contain functions that are called mapping the this -pointer to the object itself using apply . However, I find it inconvenient to use the this -pointer everytime I want to access other functions or properties of the object, especially because in lots of cases I use the new -operator to use function-objects the way you would use classes. I would prefer writing new Line() instead if new this.Line() .

It would be great if you could add local variables to a function the way php does it with extract (pseudocode following, it's a little more complicated)

var sample_object = {
  "some_function": function() {}
}

test() {
  extract(sample_object);
  some_function();   // imported from sample_object
}

Is that even possible?

Answer 1

I'm pretty sure eval is your only answer; but you need to be aware that if there's any input outside of your control involved, it isn't safe

function dynamicArgs (varName, varValue) {
  eval("var " + varName + "=" + JSON.encode(varValue) );
  alert(a);
}

dynamicArgs("a", "value");

You can see the problem with this. How is your function supposed to call the dynamic variable if it doesn't know its name? I hardcoded it to the a variable since I pass it in when calling it, but that's not a good solution. The only solution would be another eval . You really should think about what you need to do and whether this is useful. But it's doable.

Here it is in action: http://jsfiddle.net/mendesjuan/GG3Wu/

function dynamicArgs (varName, varValue) {
  eval('var ' + varName + "='" + varValue + "';");
  alert(eval(varName));
}

dynamicArgs("f", "Here I am");

Now here's an example like what you're doing, creating a variable from this.MyConstructor http://jsfiddle.net/mendesjuan/AK3WD/

var ns = {
    MyConstructor: function(val) {
       this.prop = val;
    },

    runConstructor: function(val) {
      var Ctor = "MyConstructor";
      eval('var ' + Ctor + ' =  this.' + Ctor);
      return new MyConstructor(val);
    }
}


alert( ns.runConstructor("Hello").prop );

And here's an example if you wanted to import all the values from an object into the scope;

http://jsfiddle.net/mendesjuan/AK3WD/1/

var ns = {
    MyConstructor: function(val) {
       this.val= val;
    },

    anotherProperty: 5,

    runConstructor: function(val) {
        // Bring all the variables from this into this scope
        for (var prop in this) {
            eval('var ' + prop + ' =  this.' + prop);
        }
        alert('Testing var anotherProperty: ' + anotherProperty);
        var obj =  new MyConstructor(val);
        alert('Created MyConstructor: its prop is ' + obj.val)
    }
}


ns.runConstructor("Hello");

Answer 2

There is controversial with , which has some great applications , but is marginally slow and prone to errors. It throws an error in the strict mode (which you should always opt into) and is going to be deprecated.

var sampleObject = {
  someFunction: function() {},
  b: 10
}

with (sampleObject) {
    typeof someFunction // "function"

    var a = 42
    var b = 20
}

sampleObject.a // undefined
sampleObject.b // 20

Note, that new variables defined in a with -block won't be added to the object. Nevertheless, if the object already had an eponymous property in it, this property would be modified (thanks, @Rocket).

Just for fun , here's an implementation of extract using eval (which is even more evil than with ). You can do unspeakable things with it, for example if your object has properties like sampleObject['x; while (true) { alert("Hi!") }'] sampleObject['x; while (true) { alert("Hi!") }'] .

Answer 3

This is how I did it:

   function smObject ( object) {
    return function () {
        function getter(prop) {
            return function() {
                return this[prop];
            }
        }

        function setter(prop) {
            return function(data) {
                this[prop]=data;
            }
        }

        for (var o = 0; o < object.length; o++) {
            this[object[o]] = {};
            this['get' + object[o]] = getter(object[o]);
            this['set' + object[o]] = setter(object[o]);
        }
    }
}

now you can instantiate a function like this:

var fields = ['Name', 'Id', 'Other', '....' ]
var MyFunction = smObject( fields );
var myObject = new MyFunction();

// getter/setters
myObject.setId(5);
myObject.getId(); // will return 5

Regards, Emanouil