Why should I use a semicolon after every function in javascript?

Semicolons after function declarations are not necessary.

The grammar of a FunctionDeclaration is described in the specification as this:

function Identifier ( FormalParameterListopt ) { FunctionBody }

There's no semicolon grammatically required, but might wonder why?

Semicolons serve to separate statements from each other, and a FunctionDeclaration is not a statement.

FunctionDeclarations are evaluated before the code enters into execution, hoisting is a common word used to explain this behaviour.

The terms "function declaration" and "function statement" are often wrongly used interchangeably, because there is no function statement described in the ECMAScript Specification, however there are some implementations that include a function statement in their grammar, -notably Mozilla- but again this is non-standard.

However, semicolons are always recommended where you use FunctionExpressions. For example:

var myFn = function () {
  //...
};

(function () {
  //...
})();

If you omit the semicolon after the first function in the above example, you will get completely undesired results:

var myFn = function () {
  alert("Surprise!");
} // <-- No semicolon!

(function () {
  //...
})();

The first function will be executed immediately, because the parentheses surrounding the second one will be interpreted as the Arguments of a function call.

Recommended lectures:

Named function expressions demystified (great article)

Explain JavaScript’s encapsulated anonymous function syntax (more on FunctionDeclaration vs FunctionExpression)

I use them after function-as-variable declarations:

var f = function() { ... };

but not after classical-style definitions:

function f() {
    ...
}

JS Lint is de-facto convention, and it says no semicolon after function body. See the "Semicolon" section.

Just stay consistent! They are not needed, but I personally use them because most minification techniques rely on the semi-colon (for instance, Packer).

Really just depends on your preference. I like to end lines of code with semi colons because I'm used to Java, C++, C#, etc, so I use the same standards for coding in javascript.

I don't typically end function declarations in semi colons though, but that is just my preference.

The browsers will run it either way, but maybe some day they'll come up with some stricter standards governing this.

Example of code I would write:

function handleClickEvent(e)
{
     // comment
     var something = true;  // line of code
     if (something)  // code block
     {
        doSomething();  // function call
     }
}

It's actually more than an issue of convention or consistency.

I'm fairly certain that not placing semicolons after every statement slows down the internal parser because it has to figure out where the end of the statement is. I wish I had some handy numbers for you to positively confirm that, but maybe you can google it yourself. :)

Also, when you are compressing or minifying code, a lack of semi-colons can lead to a minified version of your script that doesn't do what you wanted because all the white space goes away.

When I minified my scripts I realized that I need to use semicolon for functions which starts with equals mark. if you define a function as var, yes you need to use semicolon.

need semicolon

var x = function(){};
var x = new function(){};
this.x = function(){};

no need semicolon

function x(){}

the semicolon after a function is not necessary using it or not, does not cause errors in your program. however, if you plan to minify your code, then using semicolons after functions is a good idea. say for example you have code like the one below

//file one
var one=1;
var two=2;
function tryOne(){}
function trytwo(){}

and

//file two
var one=1;
var two=2;
function tryOne(){};
function trytwo(){};

when you minify the both, you will get the following as output

Note that comments are just for ilustration

//file one
var one=1;var two=2;function tryOne(){}
function trytwo(){}

and

//file two
var one=1;var two=2;function tryOne(){};function trytwo(){};

Edit: It doesn't matter with ECMAScript 2021 (ES2021) so please ignore the below statement.

It is a good practice to leave the semicolons ; after the end of function braces. They have been considered a best practice.

One advantage of always using them is if you want to minify your JavaScript.

As minifying the Javascript, helps to reduce the file size a bit.

But as for the best practise and answer above, not recommended to use it after a function tag.