How to set default boolean values in JavaScript?

How about:

this.hasWheels = (typeof hasWheels !== 'undefined') ? hasWheels : true;

Your other option is:

this.hasWheels = arguments.length > 0 ? hasWheels : true;

There are variations to be noted of from posted answers.

var Var = function( value ) {
    this.value0 = value !== false;
    this.value1 = value !== false && value !== 'false';
    this.value2 = arguments.length <= 0 ? true : arguments[0];
    this.value3 = arguments[0] === undefined ? true : arguments[0];
    this.value4 = arguments.length <= 0 || arguments[0] === undefined ? true : arguments[0];
};

                     value0   value1   value2        value3         value4
---------------------------------------------------------------------------
Var("")              true     true     true          true           true
Var("''")            true     true     ''            ''             ''
Var("0")             true     true     0             0              0
Var("'0'")           true     true     '0'           '0'            '0'
Var("NaN")           true     true     NaN           NaN            NaN
Var("'NaN'")         true     true     'NaN'         'NaN'          'NaN'
Var("null")          true     true     null          null           null
Var("'null'")        true     true     'null'        'null'         'null'
Var("undefined")     true     true     undefined     true           true
Var("'undefined'")   true     true     'undefined'   'undefined'    'undefined'
Var("true")          true     true     true          true           true
Var("'true'")        true     true     'true'        'true'         'true'
Var("false")         false    false    false         false          false
Var("'false'")       true     false    'false'       'false'        'false'

• value1 is made especially from value0 for string 'false' if one needs it to be boolean false. I found this relaxation useful occationally.
• value2 and value3 are modifications of original posted answers for consistency, without changed results.
• value4 is how Babel compiles for default parameters.

You can do this:

this.hasWheels = hasWheels !== false;

That gets you a true value except when hasWheels is explicitly false. (Other falsy values, including null and undefined, will result in true, which I think is what you want.)