“var” or no “var” in JavaScript’s “for-in” loop?

by

Use var, it reduces the scope of the variable otherwise the variable looks up to the nearest closure searching for a var statement. If it cannot find a var then it is global (if you are in a strict mode, using strict, global variables throw an error). This can lead to problems like the following.

function f (){
    for (i=0; i<5; i++);
}
var i = 2;
f ();
alert (i); //i == 5. i should be 2

If you write var i in the for loop the alert shows 2.

JavaScript Scoping and Hoisting

More Related Contents:

  1. JavaScript Syntax error
  2. Location of parenthesis for auto-executing anonymous JavaScript functions?
  3. What do parentheses surrounding an object/function/class declaration mean? [duplicate]
  4. How does this JavaScript/jQuery syntax work: (function( window, undefined ) { })(window)?
  5. What does the leading semicolon in JavaScript libraries do?
  6. Why avoid increment (“++”) and decrement (“–“) operators in JavaScript?
  7. What does a tilde do when it precedes an expression?
  8. How do I convert a float number to a whole number in JavaScript?
  9. Javascript: Get deep value from object by passing path to it as string [duplicate]
  10. JavaScript Loops: for…in vs for
  11. Is && statement() the same as if() statement()?
  12. How can I concatenate regex literals in JavaScript?
  13. Javascript Property with three dots (…)
  14. Advanced JavaScript: Why is this function wrapped in parentheses? [duplicate]
  15. difference between dot notation and bracket notation in javascript [duplicate]
  16. When should I use a semicolon after curly braces?
  17. Use of commas versus semicolons?
  18. JavaScript error (Uncaught SyntaxError: Unexpected end of input)
  19. Difference between var and this in Javascript functions?
  20. “elseif” syntax in JavaScript
  21. What is the decimal separator symbol in JavaScript?
  22. How can % signs be used in identifiers
  23. What is “?:” notation in JavaScript?
  24. Javascript while loop return value
  25. ES6+ javascript module export options
  26. javascript syntax: function calls and using parenthesis
  27. JavaScript dot notation [duplicate]
  28. Using a block’s return value in JavaScript
  29. Javascript history.PushState not working?
  30. + operator before expression in javascript: what does it do?

Leave a Comment