JavaScript IIFE 评估未定义

Question

Hello I'ma beginner at JavaScript and I'm trying to figure out how IIFE work, and I still have some trouble. In the lecture I stumbled upon this example code, and the evaluation of the code below gives the result „undefined“ whereas calling the function with „add()“ returns 1 and I really don't understand why, since function is an IIFE.

var add = (function() {
       var counter = 0;

       return function() {
            counter += 1;
            return counter 
       }
})();
//Output: undefined

Answer 1

You IIFE is a factory function . In this case it returns a function, so that works. You can assign the result to a variable and subsequently run the resulting function. The counter variable is ' closed over ', it will be manipulated (and returned) via the returned function, but it will not be accessible to the 'outside world'.

 // assign the result to a variable const add = (function() { let counter = 0; return function() { counter += 1; return counter; } })(); // add is a function: execute it a few times add(); add(); console.log(add()); // => 3

Answer 2

There's nothing wrong with your IIFE. You are only seeing the result of the var statement. Examples:

var x = function() {};
//=> undefined
var x = 42;
//=> undefined
var x = true;
//=> undefined

An assignment expression will evaluate to the value on the right:

y = 10;
//=> 10

But the var statement itself returns nothing:

var y = 10;
//=> undefined

Relevant part of the spec:

VariableDeclaration: BindingPattern Initializer

1. Let rhs be the result of evaluating Initializer.
2. Let rval be? GetValue(rhs).
3. Return the result of performing BindingInitialization for BindingPattern passing rval and undefined as arguments.

See https://tc39.es/ecma262/#sec-variable-statement

Answer 3

For IIFE you need to wrap your anonymous function in "( )" and then invoke it with "();"It does not allow declaration. try removing declaration.

(function() {
   var counter = 0;

   return function() {
        counter += 1;
        return counter 
   }
})();

For more info refer this. https://www.tutorialsteacher.com/javascript/immediately-invoked-function-expression-iife