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# undefined value in JS

In Javascript, `undefined` does not mean some empty value or undeclared variable.

If a variable has a value `undefined`, it means it has been declared in the code, it has been hoisted in the memory space/execution context but has not been assigned any value. Thus `undefined` is a special value in javascript, all variables declared in the code are assigned this undefined value in the 1st Hoisting phase of code execution cycle. ( Javascript code is executed in two phases - see Javascript Code Execution topic).

If your code uses some *undeclared* value, the javascript execution will throw an error `'Uncaught Reference Error: <<var>> is not declared'`

Since `undefined` is a special variable/value in javascript, we can write boolean expressions comparing a variable to `undefined`.

```
console.log(a)

/// ------- output
/// above code would trigger an error when the code runs
/// Uncaught Reference Error: a is not declared
```

```
var a;
console.log('>>>>', a)

if (a === undefined) {
 console.log('>>>> a has a value of undefined');
} else {
 console.log('>>>> a has some defined value');
}

/// ------- output
/// above code would output below
/// >>>> undefined
/// >>>> a has a value of undefined

```

When an undeclared variable is assigned a value, the JS engine (in non-strict mode) would automatically add the variable to the global object in the 1st **Hoisting** phase of  code execution.&#x20;

```
a1 = 3;
console.log('a1=', a1)

/// ------- output
/// for the above code  (assuming 'use strict' is not enabled),
/// the JS engine (in non-strict mode), would add 'a' to the global object
/// a1=3


/// in strict mode, for the above the JS engine will throw 
/// Uncaught Reference Error: a1 is not declared

```

{% embed url="<https://youtu.be/AsVkKqqT8S0>" %}