Why Does JavaScript's `map` Method Fail on Arrays Created with `new Array(count)`?

Understanding Array Creation and map Method Behavior

The map method is a powerful tool in JavaScript for transforming elements in an array. However, you may encounter unexpected results when using it on arrays created with new Array(count).

Array Creation

The new Array(count) syntax creates an array with a specified number of elements. However, these elements are not initialized with any default values, resulting in an array filled with undefined values.

map Method Behavior

The map method applies a function to each element in an array and returns a new array with the modified elements. In the case of an array created with new Array(count) and populated with undefined values, the map method does not alter the values because undefined does not evaluate to truthy in JavaScript.

Real-World Scenario

Consider the following code:

var x = new Array(3);
console.log(x.map(function() { return 0; }));

This code creates an array with three undefined elements. When you apply the map method, it does not change the elements, and the output remains an array with undefined values.

Solution

To fix this issue, you can fill the array with any value (even undefined) using Array.prototype.fill(). This ensures that the map method has values to work with.

var x = new Array(3).fill(undefined);
console.log(x.map(function() { return 0; })); // [0, 0, 0]

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