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+# Chapter 8. Functions
+
+**Function** = a block of JS code that's defined once but may be invoked multiple times.
+* *argument*: values provided by during invocation as function's parameters.
+* *return value*: computed by function using argument values
+* *invocation context*: value of `this` keyword for each invocation
+
+Utilization of functions:
+* **method of object**: if function is assigned to a property of an obj.
+* When a function is invoked on an object (e.g. `obj.method()`), this object become invocation context (aka `this`) for the function
+* **Constructor**: function designed to initialize a new obj.
+
+Function as **first class citizen** (object):
+* In JS, functions are obj. Hence, can be manipulated by programs
+* We can set properties on functions and invoke methods on them (i.e. pass function as parameter to method)
+
+JS function definitions can be nested within other functions
+
+## 8.1 Defining Functions
+
+### 8.1.1 Function Declarations
+
+**Function declaration** = `function` keyword + Identifier as function name + `(param1, param2, ...)` + `{JS statements as function body}`
+* `return` causes function to stop executing and return computed value to caller
+* if body has no `return`, value of function is `undefined`
+
+```js
+// Print the name and value of each property of o.  Return undefined.
+function printprops(o) {
+    for(let p in o) {
+        console.log(`${p}: ${o[p]}\n`);
+    }
+}
+
+// Compute the distance between Cartesian points (x1,y1) and (x2,y2).
+function distance(x1, y1, x2, y2) {
+    let dx = x2 - x1;
+    let dy = y2 - y1;
+    return Math.sqrt(dx*dx + dy*dy);
+}
+
+// A recursive function (one that calls itself) that computes factorials
+// Recall that x! is the product of x and all positive integers less than it.
+function factorial(x) {
+    if (x <= 1) return 1;
+    return x * factorial(x-1);
+}
+```
+
+* Name of function (in function declaration) becomes a variable, whose value is function itself.
+* function declaration statements are "hoisted" (level up) to top of enclosing block.
+  * All function in a JS block will be defined before JS interpreter start execution
+
+### 8.1.2 Function Expressions
+
+Function Expression (FE 函数表达式) vs Function Declaration (FD 函数声明):
+* FE appear within context of a larger expression
+* 
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