Question

I opened a js file that I wrote a while back and although it’s working, I thought I spotted an error. (JS is not my primary language)

I had this:

if( myvar = fieldval.match(mypattern))
  {
     //Do Stuff
  }

So I think I get it. Is this a correct statement?:
A javascript assignment operation evaluates to the value being assigned.

I tested on w3schools

<script type="text/javascript">
var str="The rain in SPAIN stays mainly in the plain"; 
var patt1=/ain/gi;
var test
document.write(test=str.match(patt1));
</script>

and it writes “ain,AIN,ain,ain” where I might have expected it to write “true” or not to write at all because boolean true is not a string. Is my line of thought and then ultimate conclusion correct. (I ask about my line of thought on this because I do not have a lot of formal CS training.)

Answer 1

It is a correct statement. The new value of myvar is tested:

if ( myvar = fieldval.match(mypattern) )

When the String.match method cannot find a match, it returns null. !!null === false, so the if-block is not evaluated. When any non-empty match is found, the condition is true, and the block is evaluated.

In this case, it is very likely that the if-statement is correct, and that the following is intended:

if ( (myvar = fieldval.match(mypattern)) !== null )