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add null undefined

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	`1`	`+`
	`2`	`+## Null and Undefined`
	`3`	`+`
	`4`	`+- Used to express 'nothing' in JavaScript.`
	`5`	`+`
	`6`	+- If we declare a variable, and don't set anything to it, then it has value `undefined`.
	`7`	`+`
	`8`	+ ```javascript
	`9`	`+ let dog;`
	`10`	`+ console.log(dog); // undefined`
	`11`	`+`
	`12`	+ ```
	`13`	`+`
	`14`	+ When we try to access a variable that is created but not defined/set a value, we get `undefined`.
	`15`	`+`
	`16`	`+- null: value of nothing`
	`17`	`+`
	`18`	+ ```javascript
	`19`	`+ const somethingNull = null;`
	`20`	`+ // we have explicitly set the value to be nothing with null`
	`21`	+ ```

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