🌟object -> primitive conversion

JS ⟩ value ⟩ type ⟩ conversion ⟩ object -> primitive

JS spec defines 3 algorithms for object -> primitive conversion：

• prefer-string：preferring a string (if conversion is possible)

• prefer-number：preferring a number (if conversion is possible)

• no-preference：classes choose their own preference.

🗺️ 圖表

🔴 主題

• prefer-string

• prefer-number

• no preference

⭐️ 重點

• object -> bool：👉 falsy values
• object -> number：👉
• object -> string：👉

  • pass an object to a built-in function that expects a string.

  • call String()
  • interpolate objects into template literal.

Date is the only built-in type that uses "prefer-string" object -> primitive conversion, all other types uses "prefer-number".

👥 相關

• 👉 Boolean / Number / String
• ⬇️ object -> primitive algorithms are involved in：

  • add/concate (+).

  • sloppy equality (==)

  • comparison operator.

• ⛔ object -> primitive algorithms may result in TypeError❗

📗 參考

• JavaScript: The Definitive Guide ⟩

  • 3.9.2 Explicit Conversions

  • 3.9.3 Object to Primitive Conversions

  • 14.4.7 (define your own object -> primitive conversion)

📘 手冊

• Object.prototype.toString() - object to string.
• Object.prototype.valueOf() - object to (primitive) value, if exists.