Why is 1 + '1' = '11' and 1 - '1' = 0 in JavaScript (Coercion)? [duplicate]

Question

This seems quite obvious in its logic (a string can't subtract) but I would like to know how this decision is taken in the underlying execution of JavaScript. How exactly are coercion rules being applied here?

Answer 1

- is defined in terms of ToNumber, whereas + has an extra clause for strings (emphasis mine):

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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