JavaScript: not defined vs ReferenceError vs TypeError

JavaScript: not defined vs ReferenceError vs TypeError

JavaScript errors have confusing names. Here are the three most common ones and the difference between them.

"not defined" (a ReferenceError)

"not defined" is the message in a ReferenceError. It means the variable does not exist in any accessible scope.

console.log(foo); // ReferenceError: foo is not defined
``
### `ReferenceError` (the error type)

A `ReferenceError` is thrown when the engine cannot resolve a variable reference. "not defined" and TDZ errors are both `ReferenceError`s:

```js
console.log(x); // ReferenceError: x is not defined (never declared)
console.log(y); let y; // ReferenceError: Cannot access 'y' before initialization (TDZ)
``
### `TypeError`

A `TypeError` is thrown when a value is not the expected type. Common cases:

- Calling a non-function:
```js
const foo = undefined;
foo(); // TypeError: foo is not a function
``
- Reading a property of `null` or `undefined`:
```js
const obj = null;
obj.name; // TypeError: Cannot read properties of null (reading 'name')
``
- Reassigning a `const`:
```js
const x = 5;
x = 10; // TypeError: Assignment to constant variable
``
### `SyntaxError`

A `SyntaxError` is thrown during parsing, before any code runs. The entire script does not execute.

```js
const x = ; // SyntaxError: Unexpected token ';'
``
### Comparison Table

| Error | When | Example |
|---|---|---|
| `ReferenceError` | Variable does not exist or is in TDZ | `console.log(foo)` (never declared) |
| `TypeError` | Value is wrong type for the operation | `undefined()` or `null.x` |
| `SyntaxError` | Code is malformed | `const x = ;` |

### The Hoisting Connection

```js
console.log(a); // undefined (var hoisted, no error)
var a = 5;

console.log(b); // ReferenceError (let in TDZ)
let b = 5;

bar(); // TypeError (var hoisted as undefined, calling undefined())
var bar = function () {};

foo(); // works (function declaration hoisted with body)
function foo() {}
``
### The Takeaway

`ReferenceError` ("not defined" or TDZ) means the variable does not exist or is not yet accessible. `TypeError` means the value is the wrong type for the operation (calling a non-function, reading properties of null/undefined). `SyntaxError` means the code is malformed and fails at parse time.