Why don’t logical operators (&& and ||) always return a boolean result?

by

In JavaScript, both || and && are logical short-circuit operators that return the first fully-determined “logical value” when evaluated from left to right.

In expression X || Y, X is first evaluated, and interpreted as a boolean value. If this boolean value is “true”, then it is returned. And Y is not evaluated. (Because it doesn’t matter whether Y is true or Y is false, X || Y has been fully determined.) That is the short-circuit part.

If this boolean value is “false”, then we still don’t know if X || Y is true or false until we evaluate Y, and interpret it as a boolean value as well. So then Y gets returned.

And && does the same, except it stops evaluating if the first argument is false.

The first tricky part is that when an expression is evaluated as “true”, then the expression itself is returned. Which counts as “true” in logical expressions, but you can also use it. So this is why you are seeing actual values being returned.

The second tricky part is that when an expression is evaluated as “false”, then in JS 1.0 and 1.1 the system would return a boolean value of “false”; whereas in JS 1.2 on it returns the actual value of the expression.

In JS false, 0, -0, "", null, undefined, NaN and document.all all count as false.

Here I am of course quoting logical values for discussion’s sake. Of course, the literal string "false" is not the same as the value false, and is therefore true.

More Related Contents:

  1. Javascript AND operator within assignment
  2. Logical operators in JavaScript — how do you use them?
  3. What’s the difference between & and && in JavaScript?
  4. Logical operator || in javascript, 0 stands for Boolean false?
  5. Why is there no logical xor in JavaScript?
  6. How does javascript logical assignment work?
  7. The logical && and || operators in JavaScript
  8. Why is there no logical XOR?
  9. What is the max size of localStorage values?
  10. Remove array element based on object property
  11. How to detect my browser version and operating system using JavaScript?
  12. Why doesn’t equality check work with arrays [duplicate]
  13. Remove duplicate values from JS array [duplicate]
  14. How do I post form data with fetch api?
  15. Test if a data exist in Firebase
  16. What does (function (x,y){…})(a,b); mean in JavaScript?
  17. Pass Javascript variable to PHP via ajax
  18. Leaflet- marker click event works fine but methods of the class are undefined in the callback function
  19. Switch Case not showing correct results
  20. Chrome Extension: Get Page Variables in Content Script
  21. Is creating JS object with Object.create(null) the same as {}?
  22. How can I return a variable from a $.getJSON function
  23. setTimeout callback argument
  24. How do I recover the “text” from the page originating the alert, esp **after** the human user has *clicked* [dismiss] on the page’s **alert**?
  25. Fullpage.js. Adding a scroll delay
  26. What JavaScript libraries are known to use the global dollar sign: window.$?
  27. Javascript Tilde & Two’s complement
  28. Check if a timeout has been cleared?
  29. JSLint: Using a function before it’s defined error
  30. Animating backgroundColor in React Native

Leave a Comment