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Exercise 1

In the following code, what are the final length values for array1, array2, array3, array4, and array5?

let array1 = [1, 2, undefined, 4];

let array2 = [1];
array2.length = 5;

let array3 = [];
array3[-1] = [1];

let array4 = [1, 2, 3, 4, 5];
array4.length = 3;

let array5 = [];
array5[100] = 3;

Solution

  • The length of array1 is 4. The length is the highest index position that has a value, plus 1. In this case, the highest index position that has a value is 3; add 1 to that, and we get the length value of 4.

  • The length of array2 is 5. You can set the length of an array. Even if the highest index position that has a value assigned is 0, assigning a new length of 5 overrides that length. For now, you can think of the resulting array as having 5 elements of which the last 4 have a value of undefined. In actuality, the array still has only one element, but has 4 gaps at the end -- the gaps aren't real elements and take up very little memory. We'll learn more about this in the Core Curriculum.

  • The length of array3 is 0. Index positions must be non-negative integers starting from 0. Negative and non-integer indexes don't get taken into account when determining an array's length.

  • The length of array4 is 3. Contrast this with array2. When you set an array to a length that is shorter than its current length, the array gets truncated to the new length. In this example, JavaScript truncates the array by removing the last two elements, leaving a total of 3 elements.

  • The length of array5 is 101. As already noted, the length is the highest index position that has a value, plus 1. In this case, the highest index position that has a value is 100, so the length is 101.

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