我有一个JavaScript范围错误，我不知道如何解决此问题

Question

the browser is telling me in 'Task.prototype.start' 'this.clear' is undefined... Here is the class (function) and it's prototypes:

function Task(_func, _ms, _repeats, _args)
{
    if (typeof _func != 'function') throw '_func is not `function`';
    if (typeof _ms != 'number') throw '_ms is not `number`';
    if (typeof _repeats != 'number') _repeats = 0; // default: no repeats, run once. optional
    // _args is optional
    this.func = _func;
    this.ms = _ms;
    this.repeats = _repeats;
    this.args = _args;
    this.tID = 0;
    this.runCounter = 0;
}

Task.prototype.isRunning = function()
{
    return (this.tID != 0);
};
Task.prototype.runsOnce = function(){
    return (this.repeats == 0);
};
Task.prototype.clear = function()
{
    if (this.isRunning())
    {
        if (this.runsOnce())
        {
            clearTimeout(this.tID);
        }
        else
        {
            clearInterval(this.tID);
        }
    }
    this.tID = 0;
    this.runCounter = 0;
};
Task.prototype.start = function()
{
    this.clear();
    var _this = this;
    var _exec = function()
    {
        if (_this.runsOnce())
        {
            _this.func(_this.args);
            _this.clear();
        }
        else
        {
            if (_this.runCounter > 0)
            {
                _this.runCounter--;
                _this.func(_this.args);
            }
            else if (_this.runCounter == -1)
            {
                _this.func(_this.args);
            }
            else
            {
                _this.clear();
            }
        }
    };
    if (this.runsOnce())
    {
        this.runCounter = 0;
        this.tID = setTimeout(_exec, this.ms);
    }
    else
    {
        this.runCounter = this.repeats;
        this.tID = setInterval(_exec, this.ms);
    }
}

EDIT: how I use it...

Task.tasks = {};
Task.exists = function(_ID)
{
    return (_ID in Task.tasks);
}
Task.create = function(_func, _ms, _repeats, _args, _ID)
{
    if (Task.exists(_ID)) return;
    Task.tasks[_ID] = new Task(_func, _ms, _repeats, _args);
}
Task.start = function(_ID)
{
    if (!Task.exists(_ID)) return;
    Task.tasks[_ID].start();
}
Task.clear = function(_ID)
{
    if (!Task.exists(_ID)) return;
    Task.tasks[_ID].clear();
}


//test task
__task = new Task(function(a){console.log( (new Date()).getTime()+': '+a ) }, 2000, 0, '[args]');

Answer 1

If you are getting that error in the .start() method, then that is because the this value is not what it is supposed to be. And, that problem probably occurs because of the way you call the .start() method. Since you don't show us how you call the .start() method we can't help specifically on what is wrong there. But, you need to make sure that it is called with the form obj.start() where obj is a Task object.

A common mistake in this regard is to pass obj.start as a callback and not realize that it won't be called as obj.start() by whomever calls the callback.

Please include how you are calling .start() for us to help more specifically.

Answer 2

Your code is fine. Make sure you're instatiating with new :

var task = new Task(...);
task.start();

Probably you're doing:

var task = Task(...);

that would not create a proper this value causing the error.

Cheers