What does an exclamation mark before a variable mean in JavaScript

by

! is the logical not operator in JavaScript.

Formally

!expression is read as:

  • Take expression and evaluate it. In your case that’s variable.onsubmit
  • Case the result of that evaluation and convert it to a boolean. In your case since onsubmit is likely a function, it means – if the function is null or undefined – return false, otherwise return true.
  • If that evaluation is true, return false. Otherwise return true.

In your case

In your case !variable.onsubmit means return true if there isn’t a function defined (and thus is falsy), otherwise return false (since there is a function defined).

Simply put – !variable means take the truth value of variable and negate it.

Thus, if (!variable) { will enter the if clause if variable is false (or coerces to false)

In total

if (!variable.onsubmit || (variable.onsubmit() != false)) {

Means – check if variable.onsubmit is defined and truthy (thus true), then it checks if calling onsubmit returns a result that coerces to true. In a short line it checks if there is no onsubmit or it returns true.

Next time, how do I find this myself?

More Related Contents:

  1. What is the quickest way to subtract a variable from a number and set the variable?
  2. Node.js bug ? function’s return value change when stored as variable
  3. html form that passes information to a variable in javascript [closed]
  4. What is the scope of variables in JavaScript?
  5. JavaScript OR (||) variable assignment explanation
  6. Pass variables by reference in JavaScript
  7. console.log() shows the changed value of a variable before the value actually changes
  8. Declaring variables without var keyword
  9. Are Variable Operators Possible?
  10. Multiple left-hand assignment with JavaScript
  11. Why is the value of my input always empty if I store it in a variable?
  12. var name produces strange result in Javascript [duplicate]
  13. What does ${} (dollar sign and curly braces) mean in a string in JavaScript?
  14. Set Webkit Keyframes Values Using Javascript Variable
  15. Javascript “Variable Variables”: how to assign variable based on another variable?
  16. JavaScript loop variable scope
  17. Dynamic variables names in javascript
  18. Global variables in Google Script (spreadsheet)
  19. Javascript variables in HTML attributes
  20. Dynamic object property names?
  21. Append to string variable [closed]
  22. Global variables in Javascript and ESLint
  23. Why does shadowed variable evaluate to undefined when defined in outside scope?
  24. ReferenceError: variable is not defined
  25. Javascript date variable assignment
  26. Use JavaScript variable as function name?
  27. JSON to string variable dump
  28. Can a JavaScript variable be used in plain HTML?
  29. Javascript: Get access to local variable or variable in closure by its name [duplicate]
  30. Print / display a JavaScript variable’s name instead of it’s value

Leave a Comment