symbols.js

/**********************************
A symbol is a JavaScript Primitive data type
(Number, String, Boolean, null, undefined)

Every symbol value returned from Symbol() is unique.  

A symbol value may be used as an identifier for object properties; this is the data type's only purpose.
**********************************/
let log = console.log;

const s1 = Symbol();
const s2 = Symbol(123);
const s3 = Symbol('steve');

//log(s1);
//log(s2);
//log(s3);
//log(typeof s2);
//log(s3.toString() );

//log( Symbol(42) == Symbol(42) );
//log( Symbol(42) === Symbol(42) );
//
//log( Symbol.for(123) === Symbol(123) );
//log( Symbol.for(123) === Symbol.for(123) );
//log( Symbol.keyFor(s2) );
//let s5 = Symbol.for(555);
//log( Symbol.keyFor(s5) );
//
//let obj = {};
//obj['prop1'] = 'value1';
//obj['prop2'] = 'value2';
//obj[Symbol()] = 'value3';
//obj[Symbol()] = 'value3';
//obj[Symbol()] = 'value3';
//obj[Symbol()] = 'value3';
//obj['prop4'] = Symbol() ;
//
//for( let p in obj){
//    log(p, obj[p]);
//}
//log( JSON.stringify(obj) );

let s4 = Symbol.for('fred');
log(s4);    //the symbol created with the key 'fred'
//
let k4 = Symbol.keyFor(s4);
log(k4);