Discover the best way to traverse a map in Typescript: Which is the preferred method - for...in, for...of, or

Question

Discover the best way to traverse a map in Typescript: Which is the preferred method - for...in, for...of, or

Currently, I am grappling with understanding the syntax required to iterate through a map in Typescript. All the keys in this particular map are strings, while the values associated with them consist of arrays of strings.

Below is an excerpt of the sample code:

let attributeMap: Map<string, string[]> = new Map<string, string[]>();

// Sample data
let sampleKey1 = "bob";

// Filling up the map
let value: string[] = attributeMap.get(sampleKey1) || [];
value.push("clever");
attributeMap.set(sampleKey1, value);

value = attributeMap.get(sampleKey1) || [];
value.push("funny");
attributeMap.set(sampleKey1, value);


// Attempting to loop through the map
for (let key in attributeMap) {
    console.log(attributeMap.get(key));
    console.log("WE'RE IN THE MAP!");
}

console.log("done");

Upon running this code as it stands, only "done" gets displayed. Nothing from within the map is printed, and neither does the message "WE'RE IN THE MAP" appear. It seems that the for loop isn't being executed at all. Can anyone shed light on why this is happening and provide insights on how to rectify this issue?

javascript typescript dictionary for-loop

Answer 1

Answer №1

When you assign keys to Maps, the keys are not externally visible on the Map itself - they are stored internally within the Map structure and can only be accessed using specific iteration methods provided by Maps. The for..in loop is designed to iterate over properties that are directly defined on an object or its prototypes.

If you want to iterate over a Map, you should use a method like this instead:

for (const [key, value] of attributeMap.entries()) {
  console.log(key, value);
}

The for..in loop is more suitable for normal objects where key-value pairs are stored as properties directly on the object, rather than for Maps which handle data differently.

When a property is assigned directly on an object, the syntax typically looks like this:

someObj.someProp = 'someVal';

This assigns the someProp property to the someObj object.

However, Maps store their data internally within the [[MapData]] internal slot of the Map, rather than as separate properties on the Map object.

As a thought experiment, in JavaScript, it is technically possible to add a property directly to a Map instance and iterate over it with for..in (essentially treating the map as an object with own properties):

const map = new Map();
map.foo = 'bar';
for (const prop in map) {
  // ...
}

However, this goes against best practices and should be avoided. You should either utilize objects with dot and bracket notation for setting/retrieving properties (along with object methods like Object.entries), or leverage the methods provided by Maps such as Map.get and Map.set for interacting with Map data.

Answer 2

When you assign keys to Maps, the keys are not externally visible on the Map itself - they are stored internally within the Map structure and can only be accessed using specific iteration methods provided by Maps. The for..in loop is designed to iterate over properties that are directly defined on an object or its prototypes.

If you want to iterate over a Map, you should use a method like this instead:

for (const [key, value] of attributeMap.entries()) {
  console.log(key, value);
}

The for..in loop is more suitable for normal objects where key-value pairs are stored as properties directly on the object, rather than for Maps which handle data differently.

When a property is assigned directly on an object, the syntax typically looks like this:

someObj.someProp = 'someVal';

This assigns the someProp property to the someObj object.

However, Maps store their data internally within the [[MapData]] internal slot of the Map, rather than as separate properties on the Map object.

As a thought experiment, in JavaScript, it is technically possible to add a property directly to a Map instance and iterate over it with for..in (essentially treating the map as an object with own properties):

const map = new Map();
map.foo = 'bar';
for (const prop in map) {
  // ...
}

However, this goes against best practices and should be avoided. You should either utilize objects with dot and bracket notation for setting/retrieving properties (along with object methods like Object.entries), or leverage the methods provided by Maps such as Map.get and Map.set for interacting with Map data.

Discover the best way to traverse a map in Typescript: Which is the preferred method - for...in, for...of, or

Answer №1

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