# 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
*