Maps and Sets

Missing Map Keys

Provide a fallback when a map lookup is missing.

map-default `Map.get` returns `undefined` when the key is missing. The nullish coalescing operator `??` can turn that missing value into a clear fallback label.

Missing Map Keys

item
map_default.js
Replay: real traced execution (multi-file project)
const item = "milk";
const label = new Map([
  ["tea", "drink"],
  ["bread", "food"],
]).get(item) ?? "missing";

console.log("item=" + item);
console.log("label=" + label);

const item = "tea";
const label = new Map([
  ["tea", "drink"],
  ["bread", "food"],
]).get(item) ?? "missing";

console.log("item=" + item);
console.log("label=" + label);

const item = "bread";
const label = new Map([
  ["tea", "drink"],
  ["bread", "food"],
]).get(item) ?? "missing";

console.log("item=" + item);
console.log("label=" + label);

  1. item ← milk, label ← missing

    1const item→ milk = "milk"; //@item="tea", "bread"2const label→ missing = new Map([3  ["tea", "drink"],4  ["bread", "food"],5]).get(item→ milk) ?? "missing";67console.log("item=" + itemmilk);8console.log("label=" + labelmissing);
    outputitem=milk
label=missing

  1. item ← tea, label ← drink

    1const item→ tea = "tea";2const label→ drink = new Map([3  ["tea", "drink"],4  ["bread", "food"],5]).get(item→ tea) ?? "missing";67console.log("item=" + itemtea);8console.log("label=" + labeldrink);
    outputitem=tea
label=drink

  1. item ← bread, label ← food

    1const item→ bread = "bread";2const label→ food = new Map([3  ["tea", "drink"],4  ["bread", "food"],5]).get(item→ bread) ?? "missing";67console.log("item=" + itembread);8console.log("label=" + labelfood);
    outputitem=bread
label=food