d3.js - Javascript - function argument -


i learning javascript, , bit confused below usage. these question may more javascript d3 specific.

http://mbostock.github.com/d3/ex/sunburst.html

var arc = d3.svg.arc() .startangle(function(d) { return d.x; }) .endangle(function(d) { return d.x + d.dx; }) .innerradius(function(d) { return math.sqrt(d.y); }) .outerradius(function(d) { return math.sqrt(d.y + d.dy); });

so,

  1. startangle, endangle etc takes function argument. rationale being argument function rather number?
  2. in full code, no "d" defined.i see in of d3 programs. "d" for? how set or passed?

thanks.

to first question:

the d3.svg.arc object used create slices inside pie diagram. each of these slices needs different start- , end-angle displayed properly.

in d3.js not pass in both angles each slice, provide function, takes 1 argument d, corresponding data element slice , returns angles. same holds other parameters set here.

another advantage is, datum can consist of multiple properties , can decide property takes part respect visualization.

your second question

when using function parameter function, common use function expression of given form. in d3.js functions, serve parameters, passed in 1 or 2 parameters (most of time part of data, needs transformed). in function expression need (or @ least should) name parameters in order address them. of time in example d used parametername parameter holding specific data item.

it doesn't need defined elsewhere normal function parameter. if wish, rewrite in following way:

function startangleparam( d ) {  return d.x; }  var arc = d3.svg.arc() .startangle( startangleparam ) ...

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