CODEWITHSUNDEEP
javascriptfunctionlambdaexpression
RocketNuts
RocketNuts

Reputation: 11140

Javascript: difference between calling lambda function or just a direct expression?

I sometimes see JS constructs like this:

var foo = (function(){ return some_expression; })();

What can be the reason for this? Is this in any way different to the much simpler:

var foo = some_expression;

I happen to came across such an example here (the window.requestAnimFrame = ... code) but I've seen it in many others places.

I know one reason to wrap stuff in a lambda function is to keep variables local, but that doesn't apply in this particular example.

Upvotes: 3

Views: 125

Answers (1)

Lingaraju E V
Lingaraju E V

Reputation: 503

Not that there seems to be any difference in your case but before ECMAScript 5, undefined was a property of the window object and was writable, so undefined = "123" would replace its value. This means that whenever any program below this line of code would try to check something like this-

var foo;
foo === undefined // false

one would get a wrong result.

In order to avoid this, a coding pattern like this was used -

(function(foo, bar, undefined) {
  // your code here
  // undefined here will always be the correct value (i.e., undefined === undefined) regardless of it being polluted somewhere outside this block
})(foo, bar)

But, this was pre ES5. So, it won't matter anymore

Upvotes: 1

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