为什么for（;;）永远不会终止？

Question

I always wondered why for(;;) acts like a while(true) . For loops run as long as the condition in the middle is true. It looks like here the condition is '' , 0 chars long. Boolean('') clearly evaluates to false. So why does this run at all?

Answer 1

See the docs :

 for ([initialization]; [condition]; [final-expression]) 

Where condition is:

An expression to be evaluated before each loop iteration. If this expression evaluates to true, statement is executed. This conditional test is optional. If omitted, the condition always evaluates to true. If the expression evaluates to false, execution skips to the first expression following the for construct.

Note that

Boolean('')

is passing an empty string to Boolean , which is not the same as what's being done in the for condition (where there's nothing at all in the condition ).

If there does exist anything at all in condition (such as the empty string), then it's coerced to a Boolean . For example, '' evaluates to false , so no iterations run:

 for (; '';) { console.log('iteration'); } console.log('done'); 

Answer 2

Really interesting question! If we take a look at the docs , it appears that all 3 possible arguments, initialization, condition, and final expression, are optional.

for ([initialization]; [condition]; [final-expression]) {...}

Initialization doesn't really matter if it's omitted, as it's just there for convenience to declare a locally-scoped variable.

Omitting the final expression also doesn't matter much, as it's just there for convenience, to do things like increment your counter variable and whatnot.

Interestingly, however, if the condition is omitted, the condition always evaluates to true. This is somewhat counter-intuitive, in my opinion!

So the key is the omission of condition. sunglasses

That means the loop conditional check always evaluates to true, so it will keep looping forever unless someone puts a stop to the madness.