The well-known symbol @@match.

This function is also used to identify if objects have the behavior of regular expressions. For example, the methods String.prototype.startsWith(), String.prototype.endsWith() and String.prototype.includes(), check if their first argument is a regular expression and will throw a TypeError if they are. Now, if the match symbol is set to false (or a Falsy value except undefined), it indicates that the object is not intended to be used as a regular expression object.

The following code will throw a TypeError:

"/bar/".startsWith(/bar/);

// Throws TypeError, as /bar/ is a regular expression
// and Symbol.match is not modified.

However, if you set Symbol.match to false, the object will be considered as not a regular expression object. The methods startsWith and endsWith won't throw a TypeError as a consequence.

const re = /foo/;
re[Symbol.match] = false;
"/foo/".startsWith(re); // true
"/baz/".endsWith(re); // false

• Polyfill of Symbol.match in core-js
• Symbol.matchAll
• Symbol.replace
• Symbol.search
• Symbol.split
• String.prototype.match()
• RegExp.prototype[@@match]()