The RegExp[@@species] static accessor property returns the constructor used to construct copied regular expressions in certain RegExp methods.
Warning: The existence of
@@speciesallows execution of arbitrary code and may create security vulnerabilities. It also makes certain optimizations much harder. Engine implementers are investigating whether to remove this feature. Avoid relying on it if possible.
Syntax
RegExp[Symbol.species]
Return value
The value of the constructor (this) on which get @@species was called. The return value is used to construct copied RegExp instances.
Description
The @@species accessor property returns the default constructor for RegExp objects. Subclass constructors may override it to change the constructor assignment. The default implementation is basically:
// Hypothetical underlying implementation for illustration
class RegExp {
static get [Symbol.species]() {
return this;
}
}
Because of this polymorphic implementation, @@species of derived subclasses would also return the constructor itself by default.
class SubRegExp extends SubRegExp {}
SubRegExp[Symbol.species] === SubRegExp; // true
Some RegExp methods create a copy of the current regex instance before running exec(), so that side effects such as changes to lastIndex are not retained. The @@species property is used to determine the constructor of the new instance. The methods that copy the current regex instance are:
Examples
Species in ordinary objects
The @@species property returns the default constructor function, which is the RegExp constructor for RegExp objects:
RegExp[Symbol.species]; // function RegExp()
Species in derived objects
In an instance of a custom RegExp subclass, such as MyRegExp, the MyRegExp species is the MyRegExp constructor. However, you might want to overwrite this, in order to return parent RegExp objects in your derived class methods:
class MyRegExp extends RegExp {
// Overwrite MyRegExp species to the parent RegExp constructor
static get [Symbol.species]() {
return RegExp;
}
}
Or you can use this to observe the copying process:
class MyRegExp extends RegExp {
constructor(...args) {
console.log("Creating a new MyRegExp instance with args:", args);
super(...args);
}
static get [Symbol.species]() {
console.log("Copying MyRegExp");
return this;
}
exec(value) {
console.log("Executing with lastIndex:", this.lastIndex);
return super.exec(value);
}
}
Array.from("aabbccdd".matchAll(new MyRegExp("[ac]", "g")));
// Creating a new MyRegExp instance with args: [ '[ac]', 'g' ]
// Copying MyRegExp
// Creating a new MyRegExp instance with args: [ MyRegExp /[ac]/g, 'g' ]
// Executing with lastIndex: 0
// Executing with lastIndex: 1
// Executing with lastIndex: 2
// Executing with lastIndex: 5
// Executing with lastIndex: 6