Get the element of an array that has an index given by a variable in JavaScript

Question

I have the following code:

var items = [item1, item2, item3];

var index = Math.random() * items.length;

I expect the statement

items[index]

to return a random item from my array, however it always returns undefined. How do I get the item indexed by the index variable?

The Math.random() function returns a floating-point, pseudo-random number in the range 0–1 (inclusive of 0, but not 1) — Pommesloch
– Pommesloch, Commented Dec 13, 2018 at 22:26

Nina Scholz · Accepted Answer · 2018-12-13 22:30:59Z

4

You need an integer value index, because an Array is organized by positive integer values.

If you use a not integer value, the value is converted to a string (this applies to the integer value, too) and used as property accessor. An in this case, for example 1.22, the value does not exist in the array and you get undefined.

BTW, arrays are objects in Javascript, so all values could be used as key for the array.

var items = ['item1', 'item2', 'item3'];
var index = Math.floor(Math.random() * items.length);

console.log(items[index]);

edited Dec 13, 2018 at 22:30

answered Dec 13, 2018 at 22:25

Nina Scholz

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Comments

ibrahim mahrir · Accepted Answer · 2018-12-13 22:25:51Z

2

You should floor the number befaore using it as subscript:

var index = Math.floor(Math.random() * items.length);

That way you'll get integer values for index and not floating ones.

edited Dec 13, 2018 at 22:25

answered Dec 13, 2018 at 22:25

ibrahim mahrir

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1 Comment

Francisco José Letterio Over a year ago

I forgot to get an integer, my bad. Thanks

user7838288 · Accepted Answer · 2018-12-13 22:25:51Z

0

Use Math.round() to round up the index

var index = Math.round(Math.random() * (items.length - 1));

Otherwise it could point to something like 2.3 which isn't incöuded in the array

answered Dec 13, 2018 at 22:25

user7838288

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