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I have a JavaScript code block like below in my html page. When I run this by loading the page. I am getting below output on my browser console.

outer world 
Uncaught TypeError: undefined is not a function

As you can see in the code snippet, I am not executing the function named b anywhere in the code. But on running the code, the output from that function is coming along with an undefined is not a function error which I could not locate anywhere in my code block.

To add more to this scenario, there is no logs when I remove any one of the parts in the code. that is. If I remove b's initialization from the code then there are no errors and output. Also if I remove the self executing function block, there are no logs or errors. Its true that the b's initialization line is missing a semicolon. but what tempts it to provide such an output confuses me. Would you help me out to figure out a reasoning for this behaviour?

Can you please help me understand why it is happening so?

var b = function() {
  console.log('outer world');
}

(function() { 

})();

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2 Answers 2

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32

Missed a ; after declaring b. The following code is equivalent to what you have. 

var b = function() {
   console.log ('outer world');
}(function() {})();

Without the ; b becomes self-executing and takes an empty function as a parameter. After that it self-executes again; however, because b does not return a function, you get an exception.

I suggest not to skip ; until you become js ninja :). Keep b inside to avoid global scope pollution. 

(function () {
    "use strict";

    var b = function () {
        console.log("hi");
    };
    // other code
}());

Semicolon auto-insertion discussion

In case you do not want semicolons, add an operator before self-running function

var b = function () { console.log('outer world') }
;(function() { /* ... */ }())

ES6 Update: 

(() => {
  const b = () => console.log('hi')
  // ...
})()
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4 Comments

Finally, an example where Javascript doesn't automatically insert a semicolon where it looks intuitive to add it!
Anything that starts with ( or [ needs ; before it.
Yes. this helps me understand the scenario. Will you encorporate the links that Barmar provided in Rickard Staaf's answer. It helped me to understand it more.
@mithunsatheesh, stackoverflow.com/questions/2846283/… most likely lists all the cases. I did learn a few for sure. But the point is that although skipping ; can make code look a bit cleaner, it will sure take more time and anger to debug.
5

You need a semicolon after your var declaration, and by the way, please remove the space between log and (

var b = function() {
       console.log('outer world');
};
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5 Comments

can you provide an explanation when we drop the semi colon how does it give such an output. What happened in background that i dont have in my code?. I dint execute b at any point
Read @kornieff's answer. And see stackoverflow.com/questions/2846283/…
It will try to execute your defined function that outputs console.log, and send it the parameter of a new anonymous function, then at the end you are trying to execute a function that does not exist, because what you have at that point is not a function anymore.
@RickardStaaf : +1. thanks for the answer. However , kornieff's explanation was kind of what i was looking for. Sorry that could not phrase my question in a better way.
@RickardStaaf, +1 for being faster than me :)

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