javascript - 为什么 1 + '1' = '11' 和 1 - '1' = 0 在 JavaScript(强制转换)中？

Question

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Answer 1

- is defined in terms of ToNumber ，而 + 有一个额外的 clause for strings (强调我的):

11.6.1 The Addition operator ( + )

The addition operator either performs string concatenation or numeric addition.

The production

AdditiveExpression : AdditiveExpression +  MultiplicativeExpression 

is evaluated as follows:

1. Let lref be the result of evaluating AdditiveExpression.
2. Let lval be GetValue(lref).
3. Let rref be the result of evaluating MultiplicativeExpression.
4. Let rval be GetValue(rref).
5. Let lprim be ToPrimitive(lval).
6. Let rprim be ToPrimitive(rval).
7. If Type(lprim) is String or Type(rprim) is String, then
   • Return the String that is the result of concatenating ToString(lprim) followed by ToString(rprim)

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