6 January 2018

5 mins

JavaScript Types

Written by Dean Gilewicz
JS
JavaScript is a loosely typed or a dynamic language. That means you don’t have to declare the type of a variable ahead of time. The type will get determined automatically while the program is being processed.

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 JavaScript has the following built-in types:
  • string
  • number
  • boolean
  • null
  • undefined
  • object
  • symbol (es6)
You can use the typeof operator in the following way to return a string indicating the type of a variable:

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 Be warned, there is a known “bug” for checking the type of null:

See the Pen thecodelog.com - JS Types 3 by Deano (@deangilewicz) on CodePen.

 We need to use a compound condition to test for a null value:

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 Variables that have no value assigned, have the undefined value.

See the Pen thecodelog.com - JS Types 5 by Deano (@deangilewicz) on CodePen.

 We can use the following technique when checking global variables, feature checking, or when writing “polyfill” code:

See the Pen thecodelog.com - JS Types 6 by Deano (@deangilewicz) on CodePen.

 String, Number and Boolean are Primitives – they are immutable (their values are unable to be changed because they are merely values). String, Number and Boolean are Primitives – they are immutable (their values are unable to be changed because they are merely values). When attempting to apply methods to these primitives, JavaScript (behind the scenes) will automatically use an Object wrapper around the primitive value, allowing for the use of methods on the value (the instance will be shipped out for garbage collection).

See the Pen thecodelog.com - JS Types 7 by Deano (@deangilewicz) on CodePen.

 Null and undefined are also both Primitives. null represents the intentional absence of any object value. undefined represents a variable that has not been assigned a value.

See the Pen thecodelog.com - JS Types 8 by Deano (@deangilewicz) on CodePen.

 JavaScript built-in types are either falsey or truthy:

See the Pen thecodelog.com - JS Types 9 by Deano (@deangilewicz) on CodePen.