Adding strings in JavaScript "1" + - "2" == "1-2"

Question

I accidentally typed the following JavaScript statement "1" + - "2" and I have the result "1-2". I am not sure why the minus sign was treated as a string rather than causing a syntax error.

I tried to search, but I did not get the answer I wanted.

Why the minus sign was treated as a string? it there online reference I can look at? thanks

Answer 1

Simple: - "2" evaluates to -2 because unary - coerces its operand to a number, which is precisely the behavior defined in the ECMA-262 spec.

11.4.7 Unary - Operator

The unary - operator converts its operand to Number type and then negates it. Note that negating +0 produces −0, and negating −0 produces +0.

The production UnaryExpression : - UnaryExpression is evaluated as follows:

1. Let expr be the result of evaluating UnaryExpression.
2. Let oldValue be ToNumber(GetValue(expr)).
3. If oldValue is NaN, return NaN.
4. Return the result of negating oldValue; that is, compute a Number with the same magnitude but opposite sign.

Then it's just a matter of string concatenation: "1" + (-2) evaluates, unsurprisingly, to "1-2". By this point it should comes as no surprise that the + is a string concatenation (and not an addition) operator in the context because that's what the spec says.

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TL;DR

Because, as always, that's the behavior required by the spec.

Answer 2

The original

"1" + - "2"

Is parsed as

"1" + ( - "2" )

The - here converts the "2" to a number and negates it, so - "2" evaluates to -2. So this becomes:

"1" + (-2)

Here, the + causes the -2 to be converted to a string, "-2", and then does simple string concatenation.