2022 第28週 學習筆記：不只是型別

總結

此篇為影片 JS Engine fundamentals [AgentConf] 的相關筆記

筆記

typeof null

console.log(typeof null); // 'object'

參考 ECMAScript 定義：

• undefined: Any variable that has not been assigned a value has the value undefined.
• null: Primitive value that represents the intentional absence of any object value.

再搭配 v8.dev 的解說：

• undefined means “no value”.
• null means “no object value”.

參考 stackOverFlow 的討論串：

• The reasoning behind this is that null, in contrast with undefined, was (and still is) often used where objects appear. In other words, null is often used to signify an empty reference to an object.

結論：

• null 的意義乃「沒有值的物件」
• 根據 v8.dev 的圖說，首先是代表「完全沒有值的 undefined 」，接著才是代表「沒有值的物件 null」
• 大絕：ECMAScript 本身就規範了 typeof 在檢驗到 null 時要回傳 object ，參考 13.5.3 The typeof Operator

typeof number

定義：

• MDN: The JavaScript Number type is a double-precision 64-bit binary format IEEE 754 value, like double in Java or C#.
• v8.dev: ECMAScript standardizes numbers as 64-bit floating-point values, also known as double precision floating-point or Float64.
  • However, that doesn’t mean that JavaScript engines store numbers in Float64 representation all the time — doing so would be terribly inefficient!
  • Engines can choose other internal representations, as long as the observable behavior matches Float64 exactly.
  • JavaScript engines can choose an optimal in-memory representation for such numbers to optimize code (that accesses array elements by index).
• 所有在 JS 世界裡面看到的數字（包括 42 這樣的整數）都是 Float64 格式，但 JS 引擎在實作上可以選擇「不」以 Float64 格式來將數字保存在記憶體中

實際上：

• JS 引擎（至少 v8 有如此特性）在處理只有整數的運算時會比處理浮點數來的快
  • v8.dev: If both operands are represented as integers, the CPU can compute the result very efficiently. V8 has additional fast-paths for the cases where the divisor is a power of two. For values represented as floats, the computation is much more complex and takes a lot longer.
  • For small integers in the 31-bit signed integer range, V8 uses a special representation called Smi. Anything that is not a Smi is represented as a HeapObject, which is the address of some entity in memory.
• 即使都是 number 型態的資料，在實際操作上也可能會有不同的處理方式（參照 v8 引擎的 Smi 與 HeapObject 這兩種實作方式）

sensible initial value

建議使用前後連貫的值來建立物件：

• 舉例：若需要一個物件來保存浮點數，可以在建立該物件時給予一組預設的 Number.MIN_VALUE （而非預設為 0 再賦予浮點數值）
• MDN: Number.MIN_VALUE is the smallest positive number that can be represented within float precision.

參考文件

• v8.dev: The story of a V8 performance cliff in React
• YouTube: JS Engine fundamentals [AgentConf]
• MDN: null
• tc39.es: 4.4.15 null value
• tc39.es: 6.1.1 The Undefined Type
• MDN: Number
• MDN: Number.MIN_VALUE