D3 v4中d3.nest().key().map().each报错：not a function求助

Hey there! Let's break down what's going on here and how to fix that frustrating error.

The Root Cause #

In D3 v3, d3.nest().map() returned a D3-specific map object that came with built-in methods like .each(). But starting with D3 v4, the nest.map() method got a big overhaul: it now returns a plain vanilla JavaScript object instead. Plain JS objects don't have an .each() method, which is exactly why you're seeing that TypeError.

Even though you spotted .each() referenced in the uncompressed D3 v4 code, that's for the standalone d3.map() collection object—not the object returned by d3.nest().map().

Solution 1: Use Native JavaScript to Traverse the Object #

The simplest fix is to lean into native JS methods to loop through your nested data. Object.entries() is perfect here because it gives you direct access to key-value pairs:

// First create your nested object as before
const nestedData = d3.nest()
  .key(function(d) { return d.year; })
  .map(data);

// Loop through key-value pairs with native forEach
Object.entries(nestedData).forEach(([key, values]) => {
  // Your custom logic here—like the return 0 from your example
  console.log(`Processing year: ${key}, values count: ${values.length}`);
});

If you prefer, you could also use Object.keys() to get just the keys and access values via bracket notation:

Object.keys(nestedData).forEach(key => {
  const values = nestedData[key];
  // Your logic here
});

Solution 2: Convert the Object to a D3 Map (If You Prefer .each()) #

If you really want to stick with D3's .each() method, you can convert the nested object to a d3.map() instance first:

const nestedObj = d3.nest()
  .key(function(d) { return d.year; })
  .map(data);

// Convert the plain JS object to a D3 map
const d3Map = d3.map(nestedObj);

// Now you can use .each() just like you originally tried
d3Map.each(function(values, key) {
  return 0; // Your custom logic here
});

A Quick Note on D3 v4's Nest Changes #

D3 v4 streamlined the nest module to align better with native JavaScript practices. Both nest.map() and nest.entries() now return standard JS structures (objects and arrays, respectively) instead of D3's custom collections. This makes it easier to work with other JS libraries and native APIs.

内容的提问来源于stack exchange，提问作者boopbeepboop