fix(guide): simplify directory structure

guide/english/javascript/standard-objects/function/function-length/index.md

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+---
+title: Function Length
+---
+## Function Length
+
+The `length` property on the function object holds the number of arguments expected by the function when called.
+
+```javascript
+
+function noArgs() { }
+
+function oneArg(a) { }
+
+console.log(noArgs.length); // 0
+
+console.log(oneArg.length); // 1
+```
+
+### ES2015 Syntax
+
+ES2015, or ES6 as it is commonly called, introduced the rest operator and default function parameters. Both of these additions change the way the `length` property works.
+
+If either the rest operator or default parameters are used in a function declaration the `length` property will only include the number of arguments before a rest operator or a default parameter.
+
+```javascript
+function withRest(...args) { }
+
+function withArgsAndRest(a, b, ...args) { }
+
+function withDefaults(a, b = 'I am the default') { }
+
+console.log(withRest.length); // 0
+
+console.log(withArgsAndRest.length); // 2
+
+console.log(withDefaults.length); // 1
+```
+
+More Information on `Function.length` can be found on [Mozilla's MDN Docs](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/length).
+