I don't understand var a = [], i here. How can a be declared as both an array and whatever i's type is?
// Source: Javascript: The Good Parts, p. 63
Array.dim = function (dimension, initial) {
var a = [], i;
for(i = 0; i< dimension; i +=1)
{
a[i] = initial;
}
return a;
}
it means declare both (separately) - not declare them to be equal
same thing as:
var a = [];
var i;
i is a var. As this answer states clearly: the above code is the same thing as declaring both variables on separate lines. The comma separates variable declarations.
i was not declared by that line, it would become a global variable. In my opinion, the notation in your example makes it much easier to forget to declare i (or easier to delete that line and forget that i was declared there). Why not just include the var keyword in the for loop??
var statement with line-breaks after each comma...The following code:
var a = [], i;
is EXACTLY the same as this code:
var a = [];
var i;
It means:
a is an empty arrayi is an uninitialized varJavascript variables don't have types.
a is initialized to an array; i is not initialized at all.
Nothing prevents you from later writing
a = 42;
i = ["Hi", "there!"];
Javavscript variables does not have any types. Here a is initialized as an array
var a = []; //Same as var a= new Array();
and i can have any value
var i = 0;// Here it is an integer
i = "Hello";// String. Type changed to string. But it is not a good practice
var a = [], i literally means define a as an array and define i as a variable. Nothing is actually assigned to i.
For example:
var a = 5,
b = 8,
c = "string";
This basically allows you to define 3 variables without having to use the word var 3 times. Shorthand JavaScript.
Defining your variables before you use them prevent errors if the variable is not present, making functions more reliable. For example, not defining x as a variable will result in:
if(x) { /* native code */ }
Throwing an error but:
if(window.x) { /* native code */ }
Will not, because it checks the window object for x, where all global variables are stored.