JavaScript nested function prototype scope

问题:

I'm still having trouble figuring on how to manage scopes in JavaScript. In this particular example, I have a draw function containing certain properties and a function that needs to draw lines based on an array.

function Draw (canvas)
{
    this.ctx = canvas.getContext('2d');
    this.street_size = 20;
}

Draw.prototype.street = function (MAP)
{

    MAP.forEach(function (name)
    {
        this.ctx.moveTo(name.start.x,name.start.y);
        this.ctx.lineTo(name.end.x,name.end.y)
        this.ctx.stroke();
    });
}

Of course, "this.ctx" inside the forEach function returns "undefined". How can I make sure that Draw()'s variables are passed to the forEach function (without doing something like ctx = this.ctx)?

回答1:

You can use .bind ^[MDN]:

MAP.forEach(function (name) {
    this.ctx.moveTo(name.start.x,name.start.y);
    this.ctx.lineTo(name.end.x,name.end.y)
    this.ctx.stroke();
}.bind(this));

Learn more about this.

回答2:

It's common to declare the object instance variable as a new variable inside the method scope:

var self = this;
MAP.forEach(function (name) {
    self.ctx.moveTo(...

This has the advantage of allowing you to continue to use this as it would be ordinarily.

回答3:

Pass this as the second argument to forEach().

MAP.forEach(function (name)
{
    this.ctx.moveTo(name.start.x,name.start.y);
    this.ctx.lineTo(name.end.x,name.end.y)
    this.ctx.stroke();
}, this);

The second argument sets the value of this in the callback.

---

MDN forEach() docs - array.forEach(callback[, thisArg])