Jenkins assigning true to boolean variable and false to def variable

Question

Very simple and straightforward question:

Let's say we have this pipeline (I don't think there is a simpler example):

node {
    env.someEnvVariable = false

    boolean asBoolean = env.someEnvVariable ?: false
    def asDef         = env.someEnvVariable ?: false

    echo "asBoolean: $asBoolean"   // prints true because of Jenkins
    echo "asDef: $asDef"           // prints false
}

Why????

Defining a variable as boolean makes Jenkins assign it a true value but defining it as def assign it the real false value

Where is taking Jenkins that true value from?

edit: another example:

node {
    env.someEnvVariable = false
    boolean someBoolean = false
    def someVar = false

    echo "envVar: ${env.someEnvVariable}"     // prints false
    echo "someBoolean: ${someBoolean}"        // prints false
    echo "someVar: ${someVar}"                // prints false
    if (env.someEnvVariable != null) {
        someBoolean = env.someEnvVariable
        someVar = env.someEnvVariable
    }
    echo "envVar: ${env.someEnvVariable}"     // prints false
    echo "someBoolean: ${someBoolean}"        // prints true because of hack
    echo "someVar: ${someVar}"                // prints false
}

Answer 1

My assumption is that when you assign environment variable it is interpreted as string 'false'. That means in both cases you try to assign a string but assigning string to boolean variable is interpreted as true if it is not empty.

And ternary operator works the same way, you check if environment variable is true (not empty). It's not so it returns the variable itself.

Answer 2

This is due to "groovy truth": a non-empty string asserts as true.

https://docs.groovy-lang.org/latest/html/documentation/core-semantics.html#Groovy-Truth