toString() on a function

JavaScriptJunior~2 min read

What `Function.prototype.toString()` does

The toString() method returns a string representation of the function's source code.

In other words, it converts the function into a string.

Example 1 - a simple function

javascript

function sum(a, b) {
  return a + b;
}

console.log(sum.toString());

Output:

javascript

"function sum(a, b) {
  return a + b;
}"

Example 2 - an arrow function

javascript

const multiply = (a, b) => a * b;

console.log(multiply.toString());

Output:

javascript

"(a, b) => a * b"

Example 3 - an anonymous function

javascript

console.log(function(x) { return x ** 2; }.toString());

Output:

javascript

"function (x) { return x ** 2; }"

Example 4 - built-in (native) functions

If you call toString() on a built-in JavaScript function (for example, Math.max), you will not see the source code, but you will get a special string:

javascript

console.log(Math.max.toString());

Output:

javascript

"function max() { [native code] }"

This means the function is implemented at the engine level (C++), not in JS.

Example 5 - on classes and methods

javascript

class User {
  sayHi() {
    console.log('Hello!');
  }
}

console.log(User.toString());

Output:

javascript

"class User {
  sayHi() {
    console.log('Hello!');
  }
}"

Behavior per the standard (ECMAScript 2019+)

Previously, browsers returned different formats (they could strip whitespace, line breaks, and so on), but starting with ES2019 (ES10):

Function.prototype.toString() returns an exact textual copy of the source code, including whitespace, comments, and formatting.

Why this is needed

Use	Example
Debugging	You can print a function's source code to the console
Inspecting content	For example, for analysis or serialization
Metaprogramming	Generating new functions "based on" others
Built-in tools	Some frameworks use `toString()` to extract arguments or a function's body (for example, Angular before ES6)

Example 6 - use in metaprogramming

javascript

function greet(name) {
  return `Hello, ${name}`;
}

const src = greet.toString();
console.log(src); // source code

const copy = new Function('return ' + src)();
console.log(copy('Tim')); // Hello, Tim

Here we serialized the function into a string, then created a new one from it via new Function.

Important to remember

toString() returns the source code, not a "compiled" variant.
For built-in functions, "[native code]" is returned.
For class methods and generators, the format matches the declaration.
Arrow functions and methods with => are returned as is, without function.

Summary

What it does	Returns the function's source code as a string
Return type	`string`
Works with	Regular functions, arrow functions, methods, classes
For built-in functions	`function ... { [native code] }`
Standard	Since ES2019 returns the exact source text
Common uses	Debugging, code analysis, metaprogramming

Short Answer

Interview ready

Premium

A concise answer to help you respond confidently on this topic during an interview.