instanceof vs typeof in JavaScript: How They Actually Work

By Rahul — Google Frontend Engineer

typeof — Checking Primitive Types

typeof returns a string indicating the type of the operand. It works on the value itself, not the prototype chain.

The null Bug

typeof null === "object" is a bug from the first version of JavaScript (1995). It was never fixed because too much existing code depends on it. Always check for null explicitly:

instanceof — Checking the Prototype Chain

instanceof checks whether the prototype of a constructor appears anywhere in the prototype chain of an object.

When Each Fails

typeof Cannot Distinguish Objects

instanceof Fails Across Realms

Real Production Issues

• Cross-iframe instanceof: We had a component library that checked error instanceof Error. Errors thrown from an iframe failed this check. We switched to duck-typing: error && typeof error.message === 'string'
• Bundler duplicates: If two versions of a library are bundled, instanceof fails because there are two different constructor functions

The Better Approach: Object.prototype.toString

This works across realms and gives precise type information.

Best Practices

• Use typeof for primitives (string, number, boolean, undefined)
• Use Array.isArray() for arrays
• Use instanceof only when you control both the constructor and the usage context
• For libraries, prefer duck-typing over instanceof

Summary

typeof checks the value type but has the null bug and cannot distinguish objects. instanceof walks the prototype chain but breaks across iframes and bundler duplicates. Use the right tool for the right situation.