Source: https://d3js.org/d3-array/intern

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Interning values ​

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The InternMap and InternSet classes extend the native JavaScript Map and Set classes, respectively, allowing Dates and other non-primitive keys by bypassing the SameValueZero algorithm when determining key equality. d3.group , d3.rollup and d3.index use an InternMap rather than a native Map.

new InternMap(iterable, key) ​

---

js

const valueByDate = new d3.InternMap([\
  [new Date("2021-01-01"), 42],\
  [new Date("2022-01-01"), 12],\
  [new Date("2023-01-01"), 45]\
]);

Examples · Source · Constructs a new Map given the specified iterable of [key, value] entries. The keys are interned using the specified key function which defaults to object.valueOf for non-primitive values. For example, to retrieve a value keyed by a given date:

js

valueByDate.get(new Date("2022-01-01")) // 12

new InternSet(iterable, key) ​

---

js

const dates = new d3.InternSet([\
  new Date("2021-01-01"),\
  new Date("2022-01-01"),\
  new Date("2023-01-01")\
]);

Examples · Source · Constructs a new Set given the specified iterable of values. The values are interned using the specified key function which defaults to object.valueOf for non-primitive values. For example, to query for a given date:

js

dates.has(new Date("2022-01-01")) // true