The ‘IN’ Operator (for Enumerable Properties)

    // in.js
    // using the “in” operator to test for properties in objects, or indices in arrays
    //
    let log = console.log;
    if('toString' in { } ){	  // pass in the name of the property as a string
                                // window has a property named document
        log('toString exists in Object')
    }
    
    let obj = {
         prop1: 1,
         prop2: 2,
         prop3: undefined,
         prop4: null
    }
    
    delete obj.prop2;
    
    if('prop1' in obj){
         log('obj.prop1 - 1');
    } else {
         log('NO obj.prop1');
    }
    if('prop2' in obj){
         log('obj.prop2');
    } else {
         log('NO obj.prop2 - deleted');
    }
    if('prop3' in obj){
         log('obj.prop3 - undefined');
    } else {
         log('NO obj.prop3');
    }
    if('prop4' in obj){
         log('obj.prop4 - null');
    } else {
         log('NO obj.prop4');
    }
    
    let obj2 = Object.create(obj, {prop5: {value:5}});
    //obj2 will have prop1, prop3, prop4, and prop5
    
    if('prop1' in obj2){
         log('obj2.prop1 - on the prototype chain');
    } else {
         log('NO obj.prop1');
    }
    if('prop5' in obj2){
         log('obj2.prop5 - 5');
    } else {
         log('NO obj2.prop5');
    }
    
    let arr = ['a', 'b', 'c'];
    if( 0 in arr ){
         log('arr has index 0');
    } 
    if( 3 in arr ){
         log('arr has index 3');
    } else {
         log('arr has NO index 3 - array only has indices 0, 1, 2');
    }
    
    // add a property that is NOT enumerable to obj
    // properties that are NOT enumerable will not show up in for...in loops, but will show up in if/else...in statements
    Object.defineProperty(obj2, 'prop6' {
          value: 6,
          writable: false,
          configurable: false,
          enumerable: false
    });
    if( 'prop6' in obj2 ){
         log('obj2.prop6 even though it is not enumerable, configurable, or writable');
    } else {
         log('NO obj2.prop6');
    }