Why does "true" == true show false in JavaScript?

Question

MDC describes the == operator as follows:

If the two operands are not of the same type, JavaScript converts the operands then applies strict comparison. If either operand is a number or a boolean, the operands are converted to numbers if possible; else if either operand is a string, the other operand is converted to a string if possible.

With this in mind, I would evaluate "true" == true as follows:

1. Are they of the same type? No
2. Is either operand a number or boolean? Yes
3. Can we convert both to a number? No (isNaN(Number("true")) // true)
4. Is either operand a string? Yes
5. Can we convert the other operand to a string? Yes (String(true) === "true" // true)

I've ended up with the strings "true" and "true", which should evaluate to true, but JavaScript shows false.

What have I missed?

Answer 1

Because "true" is converted to NaN, while true is converted to 1. So they differ.

Like you reported, both are converted to numbers, because at least true can be (see Erik Reppen's comment), and then compared.

Answer 2

The == comparison operator is defined in ECMA 5 as:

4. If Type(x) is Number and Type(y) is String,
   return the result of the comparison x == ToNumber(y).
5. If Type(x) is String and Type(y) is Number,
   return the result of the comparison ToNumber(x) == y.
6. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
7. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).

So, "true" == true is evaluated as:

1. "true" == ToNumber(true)   (via rule 7)
2. "true" == 1
3. ToNumber("true") == 1   (via rule 5)
4. NaN == 1

===> false