JavaScript .keys()

The .keys() method returns a new map iterator object containing the keys of each element in the map, respecting the insertion order. This method enables explicit iteration using a for...of loop or the iterator’s .next() method.

Note: The iterable returned by .keys() is not reusable. Once it has been fully consumed (i.e., all elements have been iterated over), it becomes exhausted. To iterate again, a new iterator must be created by calling .keys() again on the map.

Syntax

map.keys();

Parameters:

The .keys() method does not take any parameters.

Return value:

Returns a new map iterator object containing the keys of each element in the map, in order.

Example 1: Printing Each Key

This example uses a for...of loop to iterate over the iterable object returned by .keys() and prints the keys of the ratings map:

const ratings = new Map();

ratings.set(1, 'Bad');
ratings.set(5, 'Medium');
ratings.set(10, 'Excellent');

const iterator = ratings.keys();

for (const key of iterator) {
  console.log(key);
}

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The code will produce this output:

1
5
10

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Example 2: Getting the Values

In this example, the keys are saved in the numsKeys array, and the map’s .get() method is used to retrieve and print each value:

const nums = new Map([
  [2, 'Two'],
  [4, 'Four'],
  [6, 'Six'],
  [8, 'Eight'],
]);

const numsKeys = Array.from(nums.keys());

numsKeys.forEach((key) => console.log(nums.get(key)));

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The code will produce this output:

Two
Four
Six
Eight

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Codebyte Example

This example uses the .next() method to manually iterate through the keys ​​and print them to the console:

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