{"componentChunkName":"component---src-templates-blog-post-js","path":"/JavaScript/javascript-concept-6---null-undefined-undeclared/","webpackCompilationHash":"f92dbf25810602c1889e","result":{"data":{"site":{"siteMetadata":{"title":"Web Devlog","author":"RossenaHuh","siteUrl":"https://github.com/rossenahuh","comment":{"disqusShortName":"RossenaHuh","utterances":"rossenahuh/my-dev-blog.git"},"sponsor":{"buyMeACoffeeId":"rossena"}}},"markdownRemark":{"id":"8ab80559-a46f-580a-abcd-a2f4ee573adb","excerpt":"What’s the difference between a varible that is: null, undefined or undeclared?UndeclaredUndefinedIt’s declared but it hasn’t been assigned with any values.NullNull has a value. Its value is null which is a nothing value.\nCan be used in a situation where you need to zero out of…","html":"

What’s the difference between a varible that is: null, undefined or undeclared?

\n

Undeclared

\n
const foo = foo1 + 1\n\n// foo1 is undeclared
\n

Undefined

\n

It’s declared but it hasn’t been assigned with any values.

\n
let foo\nlet obj = {}\nlet arr = [1, 2, 3]\nconst temp = function() {\n  // don't return anything\n}\n\nconsole.log(foo, obj.name, arr[5], temp()) // undefined
\n

Null

\n

Null has a value. Its value is null which is a nothing value.\nCan be used in a situation where you need to zero out of value.

\n
const valueIsNull = null\n\nconsole.log(valueIsNull) // null
\n

Check for undefined

\n
let foo\n\n// bad idea\nconsole.log(typeof foo) // undefined as a string\nconsole.log(typeof temp) // undeclared, but also returns 'undefined'\n\n// preferred way\nconsole.log(foo === undefined) // boolean true
\n

Check for null

\n
const foo = null\n\n// terrible idea\nconsole.log(typeof foo) // object\n\n// preferred way\nconsole.log(foo === null) // boolean true
","frontmatter":{"title":"JavaScript Concept 6 - Null Undefined Undeclared","date":"July 06, 2019"}}},"pageContext":{"isCreatedByStatefulCreatePages":false,"slug":"/JavaScript/javascript-concept-6---null-undefined-undeclared/","previous":{"fields":{"slug":"/JavaScript/javascript-concept-5---statement(definition)-and-expression/"},"frontmatter":{"title":"JavaScript Concept 5 - Statement(Definition) and Expression","category":"JavaScript"}},"next":{"fields":{"slug":"/JavaScript/javascript-concept-7---==-and-===/"},"frontmatter":{"title":"JavaScript Concept 7 - == and ===","category":"JavaScript"}}}}}