What happens behind the scenes with this Javascript code?

Question

(()=>{
console.log("Hello, world");
})();

How does this function work? It seems very obfuscated and I'd like a walkthrough of what's really happening.

Answer 1

The expression,

() => {
console.log("Hello, world");
}

creates an Arrow function , introduced in the ECMAScript 2015 standard, and that is immediately executed with the function call expression () at the end.

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Moreover, Arrow function definitions are actually function expressions only. So, you can omit the surrounding parens and write it as

() => {
  console.log("Hello, world");
}();

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Apart from that, you have only one expression inside the arrow function. So, you don't need to create a block and write the same as

(() => console.log("Hello, world"))();

Answer 2

This is ECMAScript 6. It creates an anonymous function, using the => (called the "arrow" or "fat arrow") operator. The function does console.log("Hello, world"); when executed. The code you posted then executes that function (the trailing (); ).

It breaks down like this:

( // wrapper for the function definition
    ()=>{ // use => to create an anonymous function
        console.log("Hello, world"); // body of the function
    } // end of the function definition
) // end of the wrapper for the function definition
(); // executes the function.

You can read more about arrow functions in this prior answer that I posted or in the Mozilla documentation .

Answer 3

it outputs a message to the web console. Here is a link to a page with the full API.