CODEWITHSUNDEEP
javascriptfunction
Godders
Godders

Reputation: 24976

JavaScript: functions passed as a parameter to an if block

In the below code how does passing bar as 

 function (n) { return n; }

to foo evaluate to true in the if block?

 function foo(bar) {

    if (bar) {
       // true
    } else {
       // false
    }
 }

This has me puzzled so any help is much appreciated.

Upvotes: 2

Views: 6380

Answers (4)

Giles Smith
Giles Smith

Reputation: 1962

It always return true, because bar is not null. if an expression within an if statement is not a logic expression (e.g. if(x <7)) then performs a check for null. If it is null it returns false, otherwise true.

In your example you have defined bar as the function {returns n;} so that is why your if statement is evaluating to true.

If bar returns a bool (true or false) then you need to call the function and get the result, rather than passing a reference to the function - this is done using parentheses:

var exampleA = bar(false); // executes function bar and returns the result (false) in exampleA
var exampleB = bar; // stores a reference to the function bar in variable exampleB

Upvotes: 0

Skilldrick
Skilldrick

Reputation: 70849

If bar is bound to an anonymous function, then it is an object. Objects are 'truthy' in JavaScript.

The only values in JavaScript that are 'falsy' are:

  • false
  • null
  • undefined
  • '' (empty string)
  • 0 (zero as a number)
  • NaN

Everything else is 'truthy', including function objects.

If you meant to call the anonymous function, you'd do if (bar(5)) which would call your anonymous function with the argument 5. Then your anonymous function would return n (which is 5 in this case). As 5 is not a falsy object, this would go to the true branch as well. Doing if (bar(0)) would got to the else branch, because 0 is falsy.

Upvotes: 6

joggink
joggink

Reputation: 397

You could use typeof() if you want to know what type it returns

Upvotes: 0

Nick Craver
Nick Craver

Reputation: 630429

Anything not null, 0, false, empty string or undefined is going to evaluate to true in an if(something) statement, this is just how weak-typing in JavaScript works.

If you want more specificity you may want to look at the typeof operator to check for the type you're expecting, or use a another stronger check like this:

if(bar === true) {

Using === checks for both value and type equivalence.

Upvotes: 5

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