# JS - Checking variable


## typeof:

```shell
typeof NaN  // 'number'
```

```shell
typeof 0  // 'number'
```

```shell
typeof undefined  // 'undefined'
```

```shell
typeof {}  // 'object'
```

```shell
typeof []  // 'object'
```

```shell
typeof null  // 'object'
```

```shell
typeof new Date // 'object'
```

```shell
typeof ''  // 'string'
```

```shell
typeof ""  // 'string'
```

```shell
typeof false // 'boolean'
```

```shell
typeof Date  // 'function'
```

```shell
var a = { "x" : 0 };
typeof a.x  // 'number'
```

```shell
var a ={ "x" : 0 };
typeof a.z  // 'undefined'
```

```shell
var a = { "x" : 0 };
typeof z.x  // 'Uncaught ReferenceError: z is not defined'
```

## Comparison Operators:

```shell
var a = NaN;
if(a){console.log('->', a)} // false
```

```shell
var a = NaN;
if(a == NaN){console.log('->', a)} // false
```

```shell
var a = '';
if(a){console.log('->', a)} // false
```

```shell
var a = ' ';
if(a){console.log('->', a)} // true
```

```shell
var a = 0;
if(a){console.log('->', a)} // false
```

```shell
var a = null;
if(a){console.log('->', a)} // false

```

```shell
var a = [];
if(a){console.log('->', a)} // true

```

```shell
var a = {};
if(a){console.log('->', a)} // true
```

```shell
var a = undefined;
if(a){console.log('->', a)} // false

var b;
if(a){console.log('->', a)} // false

var a = undefined;
var b;
if(a === b){console.log('->', a)} // true

var a = undefined;
// n is never defined
if(a == n){console.log('->', a)} // 'Uncaught ReferenceError: n is not defined'
```

```shell
// n is never defined
if(n){console.log('->', n)} // 'Uncaught ReferenceError: n is not defined'
```

```shell
// n is never defined
var a = n
if(a){console.log('->', a)} // 'Uncaught ReferenceError: n is not defined'
```