- [7.1](#functions--declarations) Use named function expressions instead of function declarations. eslint: [`func-style`](http://eslint.org/docs/rules/func-style) jscs: [`disallowFunctionDeclarations`](http://jscs.info/rule/disallowFunctionDeclarations)

> Why? Function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module! Don’t forget to name the expression - anonymous functions can make it harder to locate the problem in an Error's call stack. ([Discussion](https://github.com/airbnb/javascript/issues/794))

- [7.2](#functions--iife) Wrap immediately invoked function expressions in parentheses. eslint: [`wrap-iife`](http://eslint.org/docs/rules/wrap-iife.html) jscs: [`requireParenthesesAroundIIFE`](http://jscs.info/rule/requireParenthesesAroundIIFE)

> Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.

- [7.3](#functions--in-blocks) Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint: [`no-loop-func`](http://eslint.org/docs/rules/no-loop-func.html)

- [7.6](#es6-rest) Never use `arguments`, opt to use rest syntax `...` instead. eslint: [`prefer-rest-params`](http://eslint.org/docs/rules/prefer-rest-params)

// No! We shouldn't mutate function arguments.

// Double bad: if opts is falsy it'll be set to an object which may

// be what you want but it can introduce subtle bugs.

- [7.10](#functions--constructor) Never use the Function constructor to create a new function. eslint: [`no-new-func`](http://eslint.org/docs/rules/no-new-func)

- [7.11](#functions--signature-spacing) Spacing in a function signature. eslint: [`space-before-function-paren`](http://eslint.org/docs/rules/space-before-function-paren) [`space-before-blocks`](http://eslint.org/docs/rules/space-before-blocks)

> Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.

- [7.12](#functions--mutate-params) Never mutate parameters. eslint: [`no-param-reassign`](http://eslint.org/docs/rules/no-param-reassign.html)

> Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.

- [7.13](#functions--reassign-params) Never reassign parameters. eslint: [`no-param-reassign`](http://eslint.org/docs/rules/no-param-reassign.html)

> Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the `arguments` object. It can also cause optimization issues, especially in V8.

- [7.14](#functions--spread-vs-apply) Prefer the use of the spread operator `...` to call variadic functions. eslint: [`prefer-spread`](http://eslint.org/docs/rules/prefer-spread)

> Why? It's cleaner, you don't need to supply a context, and you can not easily compose `new` with `apply`.

- [7.15](#functions--signature-invocation-indentation) Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item.