JavaScript: if var? where var is no bool

Question

I am not understanding some piece of code of the videojs player:

if currentSrc?
    $(".video-js").replaceWith(
      "<div class='unsupported'><a href='#{currentSrc}'>Download</a></div>")

As I understand it, currentSrc has to be a bool to be checked for true or false in the if statement, but later integrated in the link it is a string. Does if var? check the var just for existence? Wouldn't that be an incorrect way to check it in JS?

In case of a string it will use the length of the string to determine true/false. a length of 0 being false and everything else being true. you can try it with this: alert(("false" ? 'true' : "false")) — Emil S. Jørgensen, Oct 12 '16 at 10:56
*currentSrc has to be a bool*, not neccarily JS will coerce the value into a bool. JS is not a typed language and will alter types based on usage — Liam, Oct 12 '16 at 10:57
@Liam this code is along the lines of `if a ? 1`, which gives `Uncaught SyntaxError: Unexpected identifier(…)` — gurvinder372, Oct 12 '16 at 10:58
Is this a duplicate of [**How does CoffeeScript's existential operator work?**](http://stackoverflow.com/q/17253772/479863)? — mu is too short, Oct 12 '16 at 16:49

score 1 · Accepted Answer · answered Oct 12 '16 at 10:56

1

currentSrc has to be a bool to be checked for true or false in the if statement

Incorrect. JavaScript is a dynamically typed language. A string of non-zero length (which any URL will be) will be a true value.

var foo = "";
var bar = "some value";

if (foo) { console.log(foo); } else { console.log("foo is false"); }
if (bar) { console.log(bar); } else { console.log("bar is false"); }

answered Oct 12 '16 at 10:56

Quentin

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So `ìf var?` is true, when var is a non-empty string and `if var` is true, when var is a string with content "true", do I get this right? – John Alba Oct 12 '16 at 11:24
Just to add clarity: this is due to the Truthy concept in JavaScript https://developer.mozilla.org/en-US/docs/Glossary/Truthy - when a string is evaluated as a boolean it will be true when it has non-zero length and false when it has 0 length. – skyline3000 Oct 12 '16 at 11:24
To correct myself: `if var?` is true when var is a non-zero length string and `if var` is true when var is a boolean with value true. – John Alba Oct 12 '16 at 11:32
But the question is CoffeeScript so `if(x)` and `if(x?)` are very different things. – mu is too short Oct 12 '16 at 16:51

Keith · Answer 2 · 2016-10-12T11:14:06.483

A nice easy way to see how javascript transforms booleans, is to use a double negation !!.

Not sure the downvote, but personally I find this technique usefull, mainly because I have Chrome open, if I'm unsure of a boolean evaluation, I just open the console and type what I'm checking.. eg. type !!-1 , in this case it would return true, some other languages might see this as false

eg.

console.log( "''", !!'' );  //false
console.log( "hello2", !!'hello' );  //true
console.log( "0", !!0 );  //false
console.log( "'0'", !!'0' );  //true
console.log( "1", !!1 ); //true
console.log( "true", !!true ); //true
console.log( "null", !!null ); //false
console.log( "undefined", !!undefined ); //false

JavaScript: if var? where var is no bool

2 Answers2