javascript：如何解决匿名函数中的“不是函数”错误？

Question

I converted a simple javascript function into one that uses a prototype. Now a call in an anonymous function is getting the "not a function" error.

From some researching, I see the value of 'this' may not always be what you think it is when an anonymous function is used.

So, how do I fix this problem?

Here's the cut-down code to show the problem.

 var chart; chart = function Chart(){ var xScale; } chart.prototype = { method1 : function() { this.xScale = d3.time.scale(); } method2 : function () { this.xScale(0); // No "not a function error here" // but I get the "not a function" error below when I call this.xScale(d.date) var overlay = d3.svg.area() .x(function (d) { return this.xScale(d.date); }) .y0(0) .y1(height); } } 

Answer 1

Within the call to d3.svg.area().x() , this is, most likely, referring to the area object. If you want to use your function(d){} , it is easiest to just create another variable referring to your current object outside of this call:

method2 : function () {
    this.xScale(0);  // No "not a function error here"

    // store the current this here
    var thisChart = this;

    var overlay = d3.svg.area()
      .x(function (d) { return thisChart.xScale(d.date); })
      .y0(0)
      .y1(height);
}

By the way, as t.niese suggests, it's not a good idea to rely on method1 begin called before method2 , to ensure that scaleX is defined.