d3.js: how to use map function?

Question

How can I use d3.map() to get [10, 12] out of the following array?

var mydata = [ 
  { '__data__' : 10 }, 
  {'__data__' : 12 }
]; 

I have been trying this, but it doesn't work:

var mymap = d3.map(mydata, function(d) { 
  return d.__data__; 
});

Answer 1

You can't -- d3.map() isn't for mapping a function across an array, but a shim for hashes. Briefly, while objects can be used like hashes, there are situations when unexpected behaviour can occur. A new Javascript standard proposes a solution to this, and until it is implemented, d3.map() can be used to the same effect.

More information in the documentation .

Answer 2

.map() just creates another array, having traversed whatever you gave it and performing a function you defined on each element in the old array and then that becomes the new array.

so if i wanted to perform a single operation on all elements in an array and make a new array out of it, i'd use .map()

for example, i need to see something represented as percentages when the data i'm getting from /proc/loadavg gives me numbers like 0.28, 0.03, 1.0, 0.97, 0.04, etc for every 5-15 min load avg. but i also need the output from running nproc in my calculation, and nproc gives me how many cores over which things are distributed. so i create an array each time /proc/loadavg gives me a new value, and i push each new value onto an array...

This would be your initial data:

var data = [0.28,0.03,1.0,0.97,0.04];
var dataSize = 5;

This could be the function to use to apply to all indexes in your original data array:

   percent = function () {
      return (loadval/(numCores*100));
   }

This would be mapping it to a new array with the transformed values:

 data = d3.range(dataSize).map(percent);