Create a single value array in JavaScript

Question

why is the following code showing undefined? Are we not allowed to create an array with a single value? Putting two values won't show this error. Is this a problem with Javascript?

<script> var tech = new Array(21); alert(tech[0]); </script> 

Answer 1

new Array(21) creates an array with a length of 21. If you want to create a single-value array, consisting of a number, use square brackets, [21]:

var tech = [ 21 ]; alert(tech[0]); 

If you want to dynamically fill an array, use the .push method:

var filler = []; for(var i=0; i<5; i++){     filler.push(i); //Example, pushing 5 integers in an array } //Filler is now equivalent to: [0, 1, 2, 3, 4] 

When the Array constructor receives one parameter p, which is a positive number, an array will be created, consisting of p elements. This feature is can be used to repeat strings, for example:

var repeat = new Array(10); repeat = repeat.join("To repeat"); //Repeat the string 9x 

Answer 2

by new Array(21) you're actually creating an array with 21 elements in it.

If you want to create an array with single value '21', then it's:

var tech = [21]; alert(tech[0]);