理解 JavaScript 中的運算子關聯性

In JavaScript, operator associativity determines the order in which operators of the same precedence level are evaluated in an expression. Operators can be either left-associative (evaluated from left to right) or right-associative (evaluated from right to left).

Left-Associative Operators

Most operators, like +, -, *, and /, are left-associative. This means that when there are multiple of these in one expression, the computer calculates from left to right.

For example:

10 - 3 - 2

This is done as (10 - 3) - 2, giving 5. The left part is calculated first.

Right-Associative Operators

Some operators, like the exponentiation operator (**), are right-associative. This means that when you see multiple ** in one line, it calculates from right to left.

For example:

2 ** 3 ** 2

This is calculated as 2 ** (3 ** 2), giving 512. The right part is calculated first.

Why Associativity Matters

• Associativity impacts complex expressions with multiple operators of the same precedence, so understanding it helps avoid unexpected results.
• Associativity does not affect operators with different precedence; those are evaluated based on precedence rules.

Summary Table

Operator	Associativity	Example
+, -, *, /	Left	10 - 3 - 2
**	Right	2 ** 3 ** 2
=, +=, -=	Right	a = b = c

Understanding associativity and precedence is crucial when writing clear and predictable JavaScript expressions.

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