Finished Chapter 8.1 Defining Functions

notes/chap8_functions.md

@@ -77,7 +77,64 @@ let tensquared = (function(x) {return x*x;}(10));
   * How FE use variable: developer can decide whether assign the newly defined function obj to a const or var, so we can refer to it mult-times later. (e.g. 3rd & 5th FE e.g. does not assign function obj to obj, and directly use it)
 * Good practice: assign FE to `const` to protect function obj.
 
-FD vs (assign FE to variable after creation):
+**FD vs. FE**:
 * Function defined by FD: the func obj are created before the script get executed (i.e. hoisted), so we can call these functions from code that appears above FD.
 * Functions defined by FE **DO NOT EXIST**  until FE are evaluated.
 * To invoke a function (either defined using FE/FD), JS must can refer to it, function defined by FE cannot be referred until it's assigned to a variable
+
+### 8.1.3 Arrow Functions
+
+In ES6, **Arrow Function** provide more compact function syntax.  **Use arrow => to separate function parameters from function body**
+
+Syntax of arrow function:
+
+* *general form*: comma-separated list of params in parentheses, followed by `=>` arrow, followed by function body in curly braces
+```js
+const func_var = (param1, param2) => { return param1 + param2; };
+```
+* *compact form (single return)*: if function body is a single `return` statement, omit `return`, semicolon, and curly braces
+```js
+const func_var = (param1, param2) => param1 + param2;
+```
+* *compact form (1 param)*: if arrow func has only 1 param, omit `()`
+```js
+const func_var = param => param*param + 2*param;
+```
+* *compact form (no param)*: if arrow func has no param, `()` must be there
+```js
+const constantFunc = () => 42;
+```
+
+Additional Syntax rules:
+* **No newline btw `(param)` and `arrow`**: it will create valid statement with other meaning (e.g. `const polynomial = x`)
+* **{} must needed in body for single return statement**: it will avoid syntactic ambiguity:
+```js
+const f = x => { return { value: x }; };  // Good: f() returns an object
+const g = x => ({ value: x });            // Good: g() returns an object
+const h = x => { value: x };              // Bad: h() returns nothing
+const i = x => { v: x, w: x };            // Bad: Syntax Error
+```
+
+#### Where to use arrow function?
+
+It's like lambda function in Python, it's ideal to pass arrow function to another function
+```js
+// Make a copy of an array with null elements removed.
+let filtered = [1,null,2,3].filter(x => x !== null); // filtered == [1,2,3]
+```
+
+#### Arrow Func vs other Funcs
+
+* Arrow func inherit value of `this` from the environment (where it's defined)
+* Arrow func do not have a `prototype` property, hence it cannot be used as constructor for new classes.
+
+### 8.1.4 Nested Functions
+
+```js
+function hypotenuse(a, b) {
+    function square(x) { return x*x; }
+    return Math.sqrt(square(a) + square(b));
+}
+```
+
+Scoping rule of nested function: enclosure function can access param and var of the functions they are nested within (i.e. inner function know outer function's param)