Description

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Advanced · 12 min

Index data with Maps

Build a lookup once when you need repeated access by identifier.

Create an index

Repeated find() calls may scan the same array many times. Build a Map for repeated ID lookups when the extra memory and setup are worthwhile. Measure real workloads before optimizing.

const users = [{ id: 1, name: "Ada" }, { id: 2, name: "Mina" }];
const byId = new Map(users.map(user => [user.id, user]));
console.log(byId.get(2).name);

Output

Mina

Know the key rules

A Map distinguishes number keys from string keys. Setting an existing key replaces its value. Decide how your application should handle duplicate IDs.

const values = new Map();
values.set(1, "First");
values.set(1, "Updated");
values.set("1", "Text key");
console.log(values.size);
console.log(values.get(1));

Output

2
Updated

Put it into practice

  1. Predict each result before running the example.
  2. Complete the coding task, then explain why your solution works.

Try it yourself

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Build an ID lookup

Index users by their numeric id, then display the name for ID 2.

Practice