Understanding Javascript immutable variable

by

Values are immutable; variables are not; they hold a reference to their (primitive) values.

The three primitive types string, number and boolean have corresponding types whose instances are objects: String, Number, Boolean.
They are sometimes called wrapper types.

The following values are primitive:

  • Strings: “hello”
  • Numbers: 6, 3.14 (all numbers in JavaScript are floating point)
  • Booleans: true, false
  • null: usually explicitly assigned
  • undefined: usually the default (automatically assigned) value

All other values are objects, including wrappers for primitives.

So:

  • Objects are mutable by default
  • Objects have unique identities and are compared by reference
  • Variables hold references to objects
  • Primitives are immutable
  • Primitives are compared by value, they don’t have individual identities

You might find The Secret Life of JavaScript Primitives a good explanation.

Also, in ES6 there is a new const keyword, that creates a read-only named constant that cannot change value through assignment or be re-declared while the script is running.

Hope this helps!

More Related Contents:

  1. How do I loop through or enumerate a JavaScript object?
  2. setTimeout calls function immediately instead of after delay
  3. Change link color of the current page with CSS
  4. Why does Javascript’s regex.exec() not always return the same value? [duplicate]
  5. Updating SVG Element Z-Index With D3
  6. Professional jQuery based Combobox control? [closed]
  7. How to register event with useEffect hooks?
  8. How do I create JavaScript array (JSON format) dynamically?
  9. Rails javascript only works after reload
  10. How do I call a Javascript function from Python?
  11. Decoding URL parameters with JavaScript
  12. can i be notified of cookie changes in client side javascript
  13. Google Protocol Buffers: JavaScript Example
  14. Are all JSON objects also valid JavaScript objects?
  15. Midpoint of route in google maps
  16. Requirejs why and when to use shim config
  17. How can I create an inset curved background that works with a background gradient and overlapping transparent shapes?
  18. Get everything after the dash in a string in JavaScript
  19. Printing emojis with JavaScript and HTML
  20. `if __name__ == ‘__main__’` equivalent in javascript es6 modules
  21. Javascript date format like ISO but local
  22. Accessing an object property with a dynamically-computed name
  23. How to append text to a div element?
  24. Loop (for each) over an array in JavaScript
  25. Disable sorting for a particular column in jQuery DataTables
  26. TypeScript React.FC confusion
  27. Javascript: Fetch DELETE and PUT requests
  28. creating objects from JS closure: should i use the “new” keyword?
  29. Using document.getElementsByName() isn’t working?
  30. Fancybox (jQuery) – Passing information from parent to iframe and iframe back to parent

Leave a Comment