JavaScript vs Java: Any Relation?

They share a name and nothing else. Brendan Eich created JavaScript at Netscape in 1995. James Gosling created Java at Sun Microsystems, and it was released in 1995 as well. The marketing team picked the name JavaScript partly because Java was popular at the time, and the name simply stuck. An email between the two camps in 1996 even joked about renaming one of them.

The two languages differ at nearly every layer. Java is statically typed and it is compiled to bytecode that runs on a virtual machine. JavaScript is dynamically typed, and every value carries a tag that says what it is while the program runs. Classes are the main way to model data in Java, while JavaScript uses objects with prototype chains and only gained class syntax in 2015. Java has threads as a first class feature that the virtual machine controls, while JavaScript gets concurrency from a single-threaded event loop, plus worker threads and web workers for heavy CPU work.

Two practical differences come up constantly in interviews. Java is case sensitive and statically typed, so a wrong type is a compile-time error. In JavaScript the same mistake appears at runtime, and often far away from the cause. Java also compares values with equals, while JavaScript's triple-equals operator compares both the type and the value without any conversion.

If somebody claims that JavaScript is a scripting version of Java, you can correct them. JavaScript did borrow some ideas from Java, but its object model and its prototype-based inheritance came from Self and from scripting languages such as Awk and Perl.

Declaring a variable with the keyword const and giving it a string works in both languages, so that will not separate them. A better test is static typing. Java asks you to declare a type for every local variable, while JavaScript works the type out at runtime.

// Valid JavaScript, rejected by javac with "cannot find symbol"
const count = 10;
console.log(count.toFixed(2));

Java needs you to write int count = 10; before the compiler will accept the variable. JavaScript accepts the declaration exactly as written, and it only complains at runtime if the value does not support the operation that you asked for.

The other direction works as well. Java has no let and no const. It only has final locals that must be assigned exactly once, and it does not have arrow functions in that syntax, so a JavaScript arrow function that is passed as a callback has no direct Java equivalent before Java 8, and even in Java 8 it becomes a lambda with a different type system. JavaScript also lets you add a property to any object while the program is running, and Java would reject that at compile time unless the field had been declared.

A strong answer mentions the practical consequence of all of this. In Java, most type mistakes are found before you ship the code. In JavaScript, they are found in production, which is why teams add type checking with JSDoc comments or with TypeScript even in projects that are otherwise plain JavaScript.