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JavaScript – hoisting, var, let, const

JavaScript – hoisting, var, let, const

Category: Front-end Łukasz Nojek

This post is a quick summary of the differences in JavaScript variable declaration.

Before ES2015 (previously named ES6), there was only one keyword to declare variables: var. The newer standard introduced two extra useful keywords: let and const.

Briefly:

  • var declares “almost global” variables
  • let declares block variables
  • const declares block read-only variables

Explanation follows.

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JavaScript: converting values to boolean or what is false and what is true

JavaScript: converting values to boolean or what is false and what is true

Category: Front-end Łukasz Nojek

What will !x return? And !!x? It depends on what is x…

!x will convert any value to a boolean, and !!x will negate the previous value. When x is a boolean, !x is its negation and !!x is back the original value x. But for other types, some magic happens thanks to auto conversion.

What exactly? I’ll explain in several cases. JavaScript recognizes 7 primitive data types, of which the most prominent ones are:

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