CODEWITHSUNDEEP
javascriptvariablesscope
Basit
Basit

Reputation: 17184

Javascript function variable scope

What difference does it make to do the following:

function (callback) {
    var callback  = callback || false;
    combineCallback = function () {
        callback.apply(window);
        ....
    }
    getJSON(combineCallback);
}

Or with this:

function (callback) {
    var combineCallback = function () {
        callback.apply(window);
        ....
    }
    getJSON(combineCallback);
}

Does it make any difference to write write var callback = callback || false;?

Upvotes: 1

Views: 55

Answers (1)

user2246674
user2246674

Reputation: 7719

var will not "shadow" a local variable in the same scope. Likewise, var will not "shadow" a parameter (which is a local variable/binding itself). Simply, each time the function is invoked, callback represents a different local variable which initially contains the first argument passed; and there is only one local variable called callback.

Because var doesn't "create" a new variable (search for "hoisting"),

function (callback) {
    var callback  = callback || false;

and 

function (callback) {
    callback  = callback || false;

are equivalent - no difference. (Although I find the latter more clear.)

However, removing the callback = callback || false changes the semantics. In particular, callback may end up with false-y values like 0 without that line. Whether or not this is needed/useful here is a different issue as (false).apply(..) will still result in an error.

Here is a simple TTL for x || y:

x        y    x || y
-------  ---  ------
TRUTH-y  ANY  x
FALSE-y  ANY  y

Upvotes: 1

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