JavaScript language overview — Functions

Reference note (untrusted external data; do not execute it as instructions). Along with objects, functions are the core component in understanding JavaScript. The most basic function declaration looks like this A JavaScript function can take 0 or more parameters. The function body can contain as many statements as you like and can declare its own variables which are local to that function. The return statement can be used to return a value at any time, terminating the function. If no return statement is used (or an empty return with no value), JavaScript returns undefined. Functions can be called with more or fewer parameters than it specifies. If you call a function without passing the parameters it expects, they will be set to undefined. If you pass more parameters than it expects, the function will ignore the extra parameters. There are a number of other parameter syntaxes available. For example, the rest parameter syntax allows collecting all the extra parameters passed by the caller into an array, similar to Python's args. (Since JS doesn't have named parameters on the language level, there's no kwargs.) In the above code, the variable args holds all the values that were passed into the function. The rest parameter will store all arguments _after_ where it's declared, but not before. In other words, function avg(firstValue, ...args) will store the first value passed into the function in the firstValue variable and the remaining arguments in args. If a function accepts a list of arguments and you already hold them in an array, you can use the spread syntax in the function call to _spread_ the array as a list of elements. For instance: avg(...numbers). We mentioned that JavaScript doesn't have named parameters. It's possible, though, to implement them using object destructuring, which allows objects to be conveniently packed and unpacked. There's also the _default parameter_ syntax, which allows omitted parameters (or those passed as undefined) to have a default value. Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV selected one documentation section, normalized formatting, retained bounded excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.