Symbols

Question 1

What is a symbols for? (2 thing)

Answer 1

identifiers for unique object properties
Symbols are always unique unless otherwise intended.
So we use a symbol to guarantee we’re creating a unique object key.
Basically so we can make properties that will be used by a different library without clashing keys.

Question 2

Symbols show up in the object properties

t or f

Answer 2

False

But they’re not private. They’re just not obvious.

Question 3

Why use symbols?

Answer 3

When an object encounters new code, it can be unsafe to have common key names applied.

The symbol as a key protects properties from being altered meeting new code.

let user = { name: "John" };
// Our script uses "id" property
user.id = "Our id value";
// ...Another script also wants "id" for its purposes...
user.id = "Their id value"
// Boom! overwritten by another script!

Question 4

Create a symbol

Answer 4

let id = Symbol()

Question 5

What does:
  let id = Symbol()
do?

Answer 5

Assigns a unique long number to id.

It’s can then be passed to an object as a unique key that we can access with id

Question 6

What is the argument for?
let id = Symbol("id");

Answer 6

It’s a description. It’s for debugging mostly

Question 7

What happens with:
let id1 = Symbol("id");
let id2 = Symbol("id");

alert(id1 == id2);

Answer 7

false

The argument is just a description. These are different symbols.

Question 8

What happens with:
let id = Symbol(“id”);
alert(id);

Answer 8

Error

Symbols don’t auto convert to strings like other data types.

Question 9

Convert a symbol to a string

Answer 9

Symbol.prototype.toString()

returns something like Symbol(‘description’

Question 10

What does:
Symbol.prototype.toString()
do?

Answer 10

Returns the symbol like this: Symbol(‘description’)

Question 11

Return a symbol’s description

Answer 11

Symbol.prototype.description

Question 12

What does:
Symbol.prototype.description
do?

Answer 12

Return a symbol’s description

Question 13

Create a hidden property on an object with a symbol

Answer 13

let user = { 
  name: "John"
};

let id = Symbol(“id”);

user[id] = 1;

Question 14

Retrieve an objects hidden property from a symbol:

let user = { 
  name: "John"
};

let id = Symbol(“id”);

user[id] = 1;

Answer 14

user[id] = 1;

Question 15

Symbols take part in For…In loops

t or f

Answer 15

false

Question 16

Symbols are copied with Object.assign()

t or f

Answer 16

True

Question 17

Symbols are returned in Object.keys()

t or f

Answer 17

false

Question 18

Create a global symbol:
// the same symbol
alert( id === idAgain ); // true

Answer 18

// read from the global registry
let id = Symbol.for("id"); // if the symbol did not exist, it is created

// read it again (maybe from another part of the code)
let idAgain = Symbol.for("id");

// the same symbol
alert( id === idAgain ); // true

Question 19

What does Symbol.prototype.for()

do?

Answer 19

Returns a specific global symbol based on the key passed into the argument
Symbol.prototype.for(key)

Question 20

Return a specific global symbol based on the key passed into the argument

Answer 20

Symbol.prototype.for(key)

Question 21

Return the symbol key for a given variable name

Answer 21

Symbol.keyFor()

let sym = Symbol.for("name");
let sym2 = Symbol.for("id");

// get name by symbol
alert( Symbol.keyFor(sym) ); // name
alert( Symbol.keyFor(sym2) ); // id

Question 22

What does
Symbol.prototype.keyFor()
do?

Answer 22

Returns the symbol key for a given variable name

let sym = Symbol.for("name");
let sym2 = Symbol.for("id");

// get name by symbol
alert( Symbol.keyFor(sym) ); // name
alert( Symbol.keyFor(sym2) ); // id

Question 23

What is a system symbol? {incomplete}

Answer 23

There exist many “system” symbols that JavaScript uses internally, and we can use them to fine-tune various aspects of our objects.