ts
letsym1 =Symbol();
letsym2 =Symbol("key");// optional string key

ts
letsym2 =Symbol("key");
letsym3 =Symbol("key");
sym2 ===sym3;// false, symbols are unique

ts
constsym =Symbol();
letobj = {
[sym]:"value",
};
console.log(obj[sym]);// "value"

ts
constgetClassNameSymbol =Symbol();
classC {
  [getClassNameSymbol]() {
return"C";
  }
}
letc =newC();
letclassName =c[getClassNameSymbol]();// "C"

unique symbol

To enable treating symbols as unique literals a special typeunique symbol is available.unique symbol is a subtype ofsymbol, and are produced only from callingSymbol() orSymbol.for(), or from explicit type annotations. This type is only allowed onconst declarations andreadonly static properties, and in order to reference a specific unique symbol, you’ll have to use thetypeof operator. Each reference to a unique symbol implies a completely unique identity that’s tied to a given declaration.

ts
declareconstsym1:uniquesymbol;
 
// sym2 can only be a constant reference.
letsym2:uniquesymbol =Symbol();
A variable whose type is a 'unique symbol' type must be 'const'.1332A variable whose type is a 'unique symbol' type must be 'const'.
 
// Works - refers to a unique symbol, but its identity is tied to 'sym1'.
letsym3:typeofsym1 =sym1;
 
// Also works.
classC {
staticreadonlyStaticSymbol:uniquesymbol =Symbol();
}Try

Because eachunique symbol has a completely separate identity, no twounique symbol types are assignable or comparable to each other.

ts
constsym2 =Symbol();
constsym3 =Symbol();
 
if (sym2 ===sym3) {
This comparison appears to be unintentional because the types 'typeof sym2' and 'typeof sym3' have no overlap.2367This comparison appears to be unintentional because the types 'typeof sym2' and 'typeof sym3' have no overlap.
// ...
}Try

Well-known Symbols

In addition to user-defined symbols, there are well-known built-in symbols.Built-in symbols are used to represent internal language behaviors.

Here is a list of well-known symbols:

Symbol.asyncIterator

A method that returns async iterator for an object, compatible to be used with for await..of loop.

Symbol.hasInstance

A method that determines if a constructor object recognizes an object as one of the constructor’s instances. Called by the semantics of the instanceof operator.

Symbol.isConcatSpreadable

A Boolean value indicating that an object should be flattened to its array elements by Array.prototype.concat.

Symbol.iterator

A method that returns the default iterator for an object. Called by the semantics of the for-of statement.

Symbol.match

A regular expression method that matches the regular expression against a string. Called by theString.prototype.match method.

Symbol.replace

A regular expression method that replaces matched substrings of a string. Called by theString.prototype.replace method.

Symbol.search

A regular expression method that returns the index within a string that matches the regular expression. Called by theString.prototype.search method.

Symbol.species

A function valued property that is the constructor function that is used to create derived objects.

Symbol.split

A regular expression method that splits a string at the indices that match the regular expression.Called by theString.prototype.split method.

Symbol.toPrimitive

A method that converts an object to a corresponding primitive value.Called by theToPrimitive abstract operation.

Symbol.toStringTag

A String value that is used in the creation of the default string description of an object.Called by the built-in methodObject.prototype.toString.

Symbol.unscopables

An Object whose own property names are property names that are excluded from the ‘with’ environment bindings of the associated objects.