Why if([]) is validated while [] == false in javascript?

by

Both current answers here are correct, but I’d like to add a more detalied explanation based on the language specification. The reason for the apparently contradictory outcomes is that if statements and equality comparisons are evaluated differently.

In the case of an if(expression) statement, the expression is evaluated and then converted to the boolean type (§ 12.5). Arrays are Objects, and when an Object is converted to Boolean, the result is always true (§ 9.2).

Equality comparisons with == follow a different set of rules, detailed on § 11.9.3. The comparison may require multiple type conversions, until both operands are the same type. The order of the operands is also important. According to that algorithm, we can see that the comparison [] == false is actually a four-step operation:

  1. There is a Boolean involved, so it’s converted to a Number first (step 7 of the algorithm). So it becomes:

    [] == 0

  2. Then the array is converted to its primitive value (see § 9.1 and § 8.12.8), and becomes an empty string (step 9). So:

    "" == 0

  3. When comparing a String to a Number, the String is converted to Number first (step 5, following the rules described on § 9.3.1):

    0 == 0
  4. Now that we have two Numbers, the comparison evaluates to true according to step 1.c.iii.

More Related Contents:

  1. What are bitwise operators?
  2. In JavaScript, why is “0” equal to false, but when tested by ‘if’ it is not false by itself?
  3. Is && statement() the same as if() statement()?
  4. Logical operator || in javascript, 0 stands for Boolean false?
  5. When to use the double not (!!) operator in JavaScript
  6. What is the purpose of new Boolean() in Javascript?
  7. In javascript, is an empty string always false as a boolean?
  8. Converting string “true” / “false” to boolean value [duplicate]
  9. Convert boolean result into number/integer
  10. How to fire jQuery function only if form is valid
  11. How to toggle a boolean?
  12. Display elements with id matching select value and hide others [closed]
  13. what I am doing wrong with javascript?
  14. Get file size, image width and height before upload
  15. Correct path for img on React.js
  16. What is the difference between window, screen, and document in JavaScript?
  17. Reading uploaded text file contents in html
  18. Is reading the `length` property of an array really that expensive an operation in JavaScript?
  19. Get IFrame’s document, from JavaScript in main document
  20. Write elements into a child iframe using Javascript or jQuery
  21. Image manipulation and texture mapping using HTML5 Canvas?
  22. How to make embedded hasMany relationships work with ember data
  23. When to prefer JSON over XML?
  24. Uploading File using Ajax in Asp.Net Core
  25. javascript syntax: function calls and using parenthesis
  26. How to store persistent data client side
  27. How to get direct url for youtube videos
  28. Error: Cannot find module ‘../lib/cli’
  29. Select2() is not a function
  30. Using non-ASCII character as JavaScript object key [duplicate]

Leave a Comment