JavaScript: What sets apart invoking a lambda function versus a regular expression?

Question

JavaScript: What sets apart invoking a lambda function versus a regular expression?

Occasionally, I come across JavaScript constructs like this:

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

Why do developers use this pattern? How is it different from the simpler alternative:

var foo = some_expression;

I recently stumbled upon such an example here (the window.requestAnimFrame = ... code) but I've seen it used in many other places as well.

I understand that one reason to wrap code in a lambda function is to maintain local scope for variables, however, that doesn't seem to be the case in this specific scenario.

javascript function lambda expression

Answer 1

Answer №1

Although there may not be a noticeable difference in your situation, prior to ECMAScript 5, undefined was actually a property of the window object and was editable. This means that assigning a new value like undefined = "123" would change its original value. As a result, any code below this point that attempted to check for undefined using something like this-

var foo;
foo === undefined // false

would produce an incorrect outcome.

To prevent such issues, developers used a coding pattern similar to the following -

(function(foo, bar, undefined) {
  // your code here
  // the value of undefined within this block always remains accurate (i.e., undefined === undefined), regardless of any changes made outside of it
})(foo, bar)

However, with the introduction of ES5, this workaround is no longer necessary.

Answer 2

Although there may not be a noticeable difference in your situation, prior to ECMAScript 5, undefined was actually a property of the window object and was editable. This means that assigning a new value like undefined = "123" would change its original value. As a result, any code below this point that attempted to check for undefined using something like this-

var foo;
foo === undefined // false

would produce an incorrect outcome.

To prevent such issues, developers used a coding pattern similar to the following -

(function(foo, bar, undefined) {
  // your code here
  // the value of undefined within this block always remains accurate (i.e., undefined === undefined), regardless of any changes made outside of it
})(foo, bar)

However, with the introduction of ES5, this workaround is no longer necessary.

JavaScript: What sets apart invoking a lambda function versus a regular expression?

Answer №1

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