[Coffeescript] Difference between with and without question mark in condition statements (判斷式中變數加不加問號的差別)

一般問號使用的時機為，如果有某物件則判斷某物件屬性 ... 等，像是

if obj?.value
  console.log "obj exists, obj.value exists, obj.value isn't 0 and obj.value isn't ''"

這邊則討論較為特別的使用情況：

console.log "obj.value? is true" if obj.value?
console.log "obj.value is true" if obj.value

以上兩行敘述，主要的差別在 obj.value 為 0 或者 "" 時會表現不一樣，

也就是情況一和二：

### Condition 1
obj =
  name: "aaa"
  value: 0

# obj.value? is true

### Condition 2
obj =
  name: "aaa"
  value: ""

# obj.value? is true

以下是兩者表現相同的情況：

### Condition 3
obj =
  name: "aaa"

# NO OUTPUT.

### Condition 4
obj =
  name: "aaa"
  value: null

# NO OUTPUT.

### Condition 5
obj =
  name: "aaa"
  value: {}

## obj.value? is true
## obj.value is true

### Condition 6
obj =
  name: "aaa"
  value: []

## obj.value? is true
## obj.value is true

轉成 Javascript 後比較清楚，分別為：

// if obj.value?
if(typeof value !== "undefined" && obj.value !== null) {
    console.log("obj.value? is true");
}

// if obj.value
if(obj.value) {
  console.log("obj.value is true");
}