Question

While reading a book about JavaScript I stumbled across an example:

var names = new Array("Paul","Catherine","Steve");
var ages = new Array(31,29,34);
var concatArray;
concatArray = names.concat(ages);

My question is, why doesn’t the variable concatArray need to be define as a new Array() in order to store the concatenated data for both arrays name and ages , but when I try to treat the concatArray as an array by adding another line of code “document.write(concatArray[0])“, it works just like an array and shows me the data stored in the first element. I just wonder why I’m not declaring the concatArray as a new array, yet it still works as one.

Answer 1

Because Javascript is dynamically typed. A variable doesn’t have a specifuc type, and an array is an object that you can assign to any variable.

When you declare a variable without assigning it a value, it just exists with an undefined value:

var answer;
// now the variable exists, but it doesn't have a value
answer = 42;
// now the variable has the numerical value 42
answer = "hello";
// now the numerical value has been replaced with the string value "hello"
answer = [];
// now the variable contains an empty array
answer[0] = 1337;
// now the variable contains an array that contains an item with the value 1337
answer = -1
// now the array is gone and the variable contains the value -1