JS: Function.prototype[Symbol.hasInstance]

By Xah Lee. Date: . Last updated: .

(new in JS: ECMAScript 2015)

f[Symbol.hasInstance](obj)
Return true if the value of f.prototype is in the Prototype Chain of obj. Else, false.

〔see Symbol Tutorial〕

The following are equivalent:

  • f[Symbol.hasInstance](obj)
  • obj instanceof f
  • f.prototype.isPrototypeOf(obj)

〔see instanceof Operator〕

// Example Symbol.hasInstance on a buildin function

const xx = [3, 4];

console.log(Array[Symbol.hasInstance](xx));
console.log(xx instanceof Array);
console.log(Array.prototype.isPrototypeOf(xx));
// Example Symbol.hasInstance on a user-defined function

function F() {}

let xx = new F();

console.log(F[Symbol.hasInstance](xx));
console.log(xx instanceof F);
console.log(F.prototype.isPrototypeOf(xx));