What is the JavaScript Map Size property?

The JavaScript Map object's size property returns the number of key/value pairs in the map. It provides an efficient way to determine the count without the need to iterate over the entire map.

It’s useful for things like:

• Validating the number of entries before performing operations.
• Checking map capacity against thresholds.
• Implementing logic that depends on the count of items in the map.

The size property of a Map object in JavaScript is read-only and reflects the number of elements in a Map. It's a straightforward way to get the count of entries, where each entry is a key-value pair.

How to use the size property

To access the size property, you simply reference it on your Map instance. Here's an example:

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let myMap = new Map();
myMap.set('key1', 'value1');
myMap.set('key2', 'value2');

console.log(myMap.size); // Outputs: 2

Size vs length

Unlike arrays, where you use length to get the number of elements, for a Map you must use size. This is because Map objects maintain the insertion order of elements and are a part of the ES6 specification.

How to Check if a Map is empty

You can check if a Map is empty by comparing its size property to 0:

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if(myMap.size === 0) {
    console.log('Map is empty');
}

Limitations and considerations

The size property is dynamic. If you add or remove items from the map, the size will update accordingly. Keep in mind that size is a property, not a method, so you do not call it with parentheses.