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>
8 Answers
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
4 Comments
davin
This is incorrect. The array doesn't contain any elements, rather it has a
length property of that value. There is a difference: a=Array(3); b=[1,2,3]; 1 in b === true; 1 in a === false
Imran Omar Bukhsh
@Rob W - but the thing is I am dynamically creating arrays in a loop, some are single valued, some are not. So whats the better way?
davin
You're still incorrect after your edit. They are not initialised as
undefined. They simply don't exist.
Rob W
@ImranOmarBukhsh Use the
.push method to fill the array. See my updated answer.
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]);
Comments
guys the answer was as simple as this:
<script>
var tech = new Array();
tech.push(21);
alert(tech[0]);
</script>
1 Comment
gxclarke
I suggest accepting the answer by @zvona as the best answer.
The above example defining an array called tech with 21 positions. You have to difine it like that
var tech = new Array('21');
alert(tech[0]);
Comments
can use push or use brackets notation , passing single value is like initialize length
var tech = new Array();
tech.push(10);
console.log(tech[0]); var tech = new Array(5);
console.log(tech.length); // length = 5
tech.push(10); // will add as a 6th element i.e. a[5]
console.log(tech.length); // length = 6
console.log(tech[0]); // undefined
console.log(tech[5]); // 10 or the easy way
var tech = [10];
console.log(tech[0]); 
Comments
Here is my solution.
var tech = new Array(); //create an empty array
tech.push(21); //Append items to the array
console.log(tech); // console array the array
Be careful to avoid this;
var array = new Array(n), this creates an empty array of length n, then if you push; you will simply append the new item to the end of the array and the new array length will be n+1 and your new array will look like this array = [Empty*21,new item ]
Comments
A possible solution to create an array using the Array constructor and having only one number element can be by using the fill method:
new Array(1).fill(4)// [ 4 ]
It will fill all the elements of the constructed array with a static value (4 in our case).
Note that fill() will only work in browsers with ES6 support.
console.log(tech.valueOf());orconsole.log(tech.toString());