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  1. 給開發者的 Web 技術文件
  2. JavaScript
  3. JavaScript 參考文件
  4. 運算式與運算子
  5. 運算子優先序
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運算子優先序

運算子優先序（Operator precedence）決定了運算子彼此之間被語法解析的方式，優先序較高的運算子會成為優先序較低運算子的運算元（operands）。

In this article

嘗試一下

console.log(3 + 4 * 5); // 3 + 20 // Expected output: 23 console.log(4 * 3 ** 2); // 4 * 9 // Expected output: 36 let a; let b; console.log((a = b = 5)); // Expected output: 5

相依性(Associativity)

當優先序相同時，使用相依性決定運算方向。範例如下：

a OP b OP c

左相依性 (Left-associativity) ，表示處理順序為從左至右 (a OP b) OP c，反之，右相依性(right-associativity) 表示處理順序為從右至左 a OP (b OP c)。賦值運算符 (Assignment operators) 為右相依性，範例如下：

js
a = b = 5;

a 和 b 的預期結果為 5，因為賦值運算符 (Assignment operator) 為右相依性，因此從右至左返回值。一開始 b 被設定為 5，接著 a 也被設定為 5。

表格(Table)

下方表格列出運算子的相依性，從高 (19) 到低 (1)。

優先性
Precedence 運算子名稱
Operator type 相依性
Associativity 運算子
Individual operators
19 Grouping 無 ( … )
18 Member Access 從左至右 … . …
Computed Member Access 從左至右 … [ … ]
new (with argument list) 無 new … ( … )
呼叫函式 從左至右 … ( … )
可選串連（Optional chaining） 從左至右 ?.
17 new (without argument list) 從右至左 new …
16 字尾遞增 無 … ++
字尾遞減 … --
15 Logical NOT 從右至左 ! …
Bitwise NOT ~ …
Unary Plus + …
Unary Negation - …
字首遞增 ++ …
字首遞減 -- …
typeof typeof …
void void …
delete delete …
await await …
14 Exponentiation 從右至左 … ** …
13 Multiplication 從左至右 … * …
Division … / …
Remainder … % …
12 Addition 從左至右 … + …
Subtraction … - …
11 Bitwise Left Shift 從左至右 … << …
Bitwise Right Shift … >> …
Bitwise Unsigned Right Shift … >>> …
10 Less Than 從左至右 … < …
Less Than Or Equal … <= …
Greater Than … > …
Greater Than Or Equal … >= …
in … in …
instanceof … instanceof …
9 Equality 從左至右 … == …
Inequality … != …
Strict Equality … === …
Strict Inequality … !== …
8 Bitwise AND 從左至右 … & …
7 Bitwise XOR 從左至右 … ^ …
6 Bitwise OR 從左至右 … | …
5 Logical AND 從左至右 … && …
4 Logical OR 從左至右 … || …
Nullish Coalescing 從左至右 … ?? …
3 條件運算 從右至左 … ? … : …
2 賦值 從右至左 … = …
… += …
… -= …
… **= …
… *= …
… /= …
… %= …
… <<= …
… >>= …
… >>>= …
… &= …
… ^= …
… |= …
… &&= …
… ||= …
… ??= …
1 Comma / Sequence 從左至右 … , …

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  1. JavaScript
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  3. JavaScript Guide
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    21. Division assignment (/=)
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    31. import()
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    34. Inequality (!=)
    35. instanceof
    36. Left shift (<<)
    37. Left shift assignment (<<=)
    38. Less than (<)
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    40. Logical AND (&&)
    41. Logical AND assignment (&&=)
    42. Logical NOT (!)
    43. Logical OR (||)
    44. Logical OR assignment (||=)
    45. 相乘運算子（*）
    46. Multiplication assignment (*=)
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    48. new.target
    49. null
    50. Nullish coalescing assignment (??=)
    51. Nullish coalescing operator (??)
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    53. 運算子優先序
    54. 可選串連
    55. Property accessors
    56. Remainder (%)
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    58. Right shift (>>)
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    60. Spread syntax (...)
    61. Strict equality (===)
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    63. 相減運算子（-）
    64. Subtraction assignment (-=)
    65. super
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    68. 一元負號運算子（-）
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    28. SyntaxError: await is only valid in async functions, async generators and modules
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    50. SyntaxError: invalid identity escape in regular expression
    51. SyntaxError: invalid named capture reference in regular expression
    52. SyntaxError: invalid property name in regular expression
    53. SyntaxError: invalid range in character class
    54. SyntaxError: invalid regexp group
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    87. SyntaxError: Unexpected token
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  14. Misc
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