D3 Update Pattern With Key Functions
  Duration: 15:45
Part of Course: Intermediate D3 Course
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Takeaways:

  • The D3 Data Operator produces three virtual selections - Enter, Update, and Exit
  • The Enter selection contains placeholder elements for any data that did not get bound to DOM elements
  • The Update selection contains all of the existing DOM elements that had their __data__ attributes updated
  • The Exit selection contains all of the existing DOM elements which did not have their __data__ attributes updated
  • When a D3 Data Join is done, there are only three possible scenarios: More Data Than DOM elements, Less Data Than DOM elements, or the same number of Data and DOM elements
  • The D3 Data Operator joins data and DOM elements from left to right, unless otherwise specified
  • The D3 Data Operator Key Function replaces the default by-index behavior of the data join, with a function of your choosing
  • To properly use the D3 Data Join Key Function, you need to have each DOM element and data point return a string that uniquely identifies it from all other elements
  • This way, D3 can match up the DOM elements that need to match up to the data points - this is why there needs to be uniqueness on the DOM element side and the Data side
  • Object constancy in D3 refers to how graphical elements that represent data can be tracked visually so that as the underlying data changes, the object can be tracked visually through the transition from the old data to the new data

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