Declare multiple variables in JavaScript

Question

I want to declare multiple variables in a function:

function foo() {
    var src_arr     = new Array();
    var caption_arr = new Array();
    var fav_arr     =          


        

Answer 1

Please stay away from that assignment pattern, even if you wanted to have all variables pointing to the same object.

In fact, only the first one will be a variable declaration, the rest are just assignments to possibly undeclared identifiers!

Assigning a value to an undeclared identifier (aka undeclared assignment) is strongly discouraged because, if the identifier is not found on the scope chain, a GLOBAL variable will be created. For example:

function test() {
    // We intend these to be local variables of 'test'.
    var foo = bar = baz = xxx = 5;
    typeof foo; // "number", while inside 'test'.
}
test();

// Testing in the global scope. test's variables no longer exist.
typeof foo; // "undefined", As desired, but,
typeof bar; // "number", BAD!, leaked to the global scope.
typeof baz; // "number"
typeof xxx; // "number"

Moreover, the ECMAScript 5th Strict Mode, disallows this kind of assignments. Under strict mode an assignment made to a non-declared identifier will cause a TypeError exception, to prevent implied globals.

By contrast, here is what we see if written correctly:

function test() {
    // We correctly declare these to be local variables inside 'test'.
    var foo, bar, baz, xxx;
    foo = bar = baz = xxx = 5;
}
test();

// Testing in the global scope. test's variables no longer exist.
typeof foo; // "undefined"
typeof bar; // "undefined"
typeof baz; // "undefined"
typeof xxx; // "undefined"