In my JavaScript code, and in Chrome dev tools I write:

a = b || "foo";

And get this error:

ReferenceError: b is not defined

And a is not set to "foo". I know this is a valid pattern in JavaScript. What am I missing?

In my JavaScript code, and in Chrome dev tools I write:

a = b || "foo";

And get this error:

ReferenceError: b is not defined

And a is not set to "foo". I know this is a valid pattern in JavaScript. What am I missing?

• javascript

• 2 "Have I not had enough coffee?" How can we know... Do you have such feeling? Than drink some more. – nicael Commented Apr 29, 2014 at 14:41
• var a = window.b || "foo";.. if b is global variable.. – Mr_Green Commented Apr 29, 2014 at 14:47
• @Mr_Green And if the variable is in another scope ? – Denys Séguret Commented Apr 29, 2014 at 14:47
• @dystroy may be this.b.. :| – Mr_Green Commented Apr 29, 2014 at 14:48
• @Mr_Green You want to thing again about this one... – Denys Séguret Commented Apr 29, 2014 at 14:50

2 Answers 2

Your pattern is OK if the value of b is undefined.

If the variable b might be not defined, it's an error to try to read it so it's a little more plicated :

a = typeof b!=="undefined" ? b : "foo";

Be careful with b||something even when you know the variable is defined (which is the most mon case) : Most often you want to provide a default value to replace undefined, not prevent the caller to pass 0 or "" so it's usually safer to do b!==undefined ? b : "foo".

That is not a valid pattern in JavaScript. It is only valid in a context where b exists, for example

function test(b) {
    var a = b || "foo";
};