JavaScript Symbols

    // A symbol is a JavaScript Primitive data type
    // Every symbol value returned from Symbol( ) is unique.
    // A symbol may be used as an identifier for object properties; this is the datatype’s only purpose
    let log = consol.log;
    const s1 = Symbol( ); 
    const s2 = Symbol(123);
    const s3 = Symbol('steve');
    
    log(s1);
    log(s2);
    log(s3);
    
    log( Symbol( ) == Symbol( ) );	// will always return false, as Symbol ( ) returns a unique value 
    log( Symbol( ) === Symbol( ) );	// will always return false, as Symbol ( ) returns a unique value
    
    log(typeof s2);	    // returns ‘symbol’
    log(s3.toString()); // returns the string "Symbol('steve')"
    
    log( Symbol.for( ) == Symbol( ) );	    // false - key of Symbol will not match description of Symbol 
    log( Symbol.for( ) == Symbol.for( ) );	// true – key of Symbol will match key of Symbol
    
    let s5 = Symbol.for(555);	// sets key to be 555
    log( Symbol.keyFor(s5) );	// returns 555, as they key for s5
    
    // OBJECTS
    let obj = { };
    obj['prop1'] = 'value1';
    obj['prop2'] = 'value2';
    obj[Symbol( )] = 'value3';
    obj['prop4'] = Symbol( );
    
    for( let p in obj) {
        log (p, obj[p]);	    // for in loop does not enumerate properties that are Symbols!
    }
    
    log(JSON.stringify(obj));	// does not return any property that has a Symbol as its key or value
    
    let s4 = Symbol.for('fred');
    log (s4);
    let k4 = Symbol.keyFor(s4); 
    log(k4);