42 lines
1.4 KiB
Markdown
42 lines
1.4 KiB
Markdown
---
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title: What's the difference between undeclared, undefined and null in JavaScript?
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shortTitle: Undeclared, undefined and null
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type: question
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language: javascript
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tags: [type]
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author: chalarangelo
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cover: river-houses
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excerpt: JavaScript has three different empty states for variables. Learn their differences and how you can check for each one.
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dateModified: 2021-06-12T19:30:41+03:00
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---
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### undeclared
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A variable is undeclared if it has not been declared with an appropriate keyword (i.e. `var`, `let` or `const`). Accessing an undeclared variable will throw a `ReferenceError`.
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```js
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console.log(x); // ReferenceError: x is not defined
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```
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### undefined
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A variable is `undefined` if it hasn't been assigned a value. `undefined` is a primitive data type in JavaScript and represents the absence of a value, intentional or otherwise.
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```js
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let x;
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console.log(x); // undefined
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```
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### null
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A variable is assigned a value of `null` like any other value. `null` is also primitive data type in JavaScript and always represents the intentional absence of a value.
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```js
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let x = null;
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console.log(x); // null
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```
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### Checking for empty states
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Undeclared variables will throw an error, which makes them easy to spot and are not very common anyways. `undefined` and `null` can be easily spotted with a conditional as they are both falsy values. Due to that, `null` and `undefined` are loosely equal (`==`), but not strictly equal (`===`).
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