Using the ternary operator with string concatenation

Question

alert("test: "+(1==2)?'hello':'world'); 

This should show me 'world' on the screen since 1 is not equal to 2.

How come it alerts 'hello'?

Answer 1

Both of the submitted answers are correct, you need to add parentheses. I figured I'd talk briefly about why.

alert("test: "+(1==2)?'hello':'world'); 

When the parser encounters a statement, it will begin recursively breaking it down into smaller and smaller chunks.

In this case, the first thing it encounters is a function: alert. Immediately the parser will look at alert's arguments, and begin parsing each of them individually. This function only has one argument, "test: "+(1==2)?'hello':'world', making it an easy first step.

At this level we can break our statement down into a series of binary comparisons. Javascript parsers form binary pairs from left-to-right (when order of operation values are the same). Our potential candidates are "test: ", (1==2), 'hello' and 'world' with operators +, ? and :. The parser will first attempt to add "test: " and (1==2). To do this it must first evaluate the statement (1==2) (which evaluates to false). The + operator causes concatenation with strings and forces all primitive variables to attempt to represent themselves as strings as well. false evaluates as the string "false" creating the statement "test: false".

The parser is now ready to evaluate the first part of the ternary: "test: false"?. In Javascript, all non-empty strings evaluate to true, passing the ternary operator's test and picking the first option "hello".

By throwing a few extra parenthesis into the original statement:

alert("test: " + ((1 == 2) ? 'hello' : 'world')); 

We tell the parser that we want to evaluate the ternary operator BEFORE concatenation.

Answer 2

Try wrapping your parens around the operation

alert("test: "+ (1 == 2 ? 'hello' : 'world')); 

demo: http://jsfiddle.net/hunter/K3PKx/

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what this is doing:

alert("test: "+(1==2)?'hello':'world'); 

is evaluating "test: " + (1==2) as true which outputs 'hello'