`x || (x = y)` pattern for using an incoming value otherwise using some default

gistfile1.js

// from https://twitter.com/#!/LeaVerou/status/16613276313980929


// this pattern below is common for using an incoming value otherwise using some default.

/* Bad:    */  x = x ? x : y; // simple ternary. 
                              // if x is truthy lets use it.

/* Better: */  x = x || y;    // logical OR. 
                              // you could string a bunch of these easily to grab the first non-falsy value: 
                              //   x = x || y || z || a;
                              
/* Best:   */  x || (x = y);  // right-hand assignment!
                              // done without the parens this would throw a syntax error.
                              // if x is truthy we don't even need to do the assigment, 
                              //   so the logical OR will bypass it.

// jslint especially hates that last one. big surprise. 



// and of course, if x is intentionally supposed to be 0, false, or '' 
//   then you're fucked and you'll have to do something nasty like:
    x = (x !== undefined) ? x : y; // yuck. (parens optional)