Thoughts caused by JavaScript array loop_javascript skills

Look at the code without talking:

Copy the code The code is as follows:

var i=arr.length ;
while(i--)
{
//Write something?
}

The above is the code of Google array.js program that suggests improvements . Think about why i-- the while loop ends when i is 0?

The following code is equivalent to it:

Copy the code The code is as follows:

var i=arr.length;
for(;i--;)
{
//i-- is written in the middle of two semicolons, is it strange? Isn't it strange?
}

Shocked? Not feeling it?

Ugh. What do we usually write between the two semicolons? This i--according to "common sense" should be after the second semicolon. Then let’s take a look at the C language code:

Copy the code The code is as follows:

int main( )
{
int i = 5;
while(i--)
{
printf("%d ", i);
}
while(1) ;
return 0;
}

The running result is also amazingly consistent with JavaScript!

Okay. I don't know why? We only know that the value 0 is converted into a Boolean value and is false:

var i= !!0;

and other values ​​are converted into a Boolean value and is true. The example program code above is an explicit conversion of a numeric value to a Boolean value. If

0 is implicitly converted to false, use the following JS program to test:

Copy code The code is as follows :

var i=0;
if(i)
{
alert('if');
}
else{
alert( 'else');
}
alert('No matter what the if and else programs are, they have to go here');

if This example code is not written in a "practical" way at all ? Well, here is a code sample snippet from a Tudou.com front-end development expert:

Copy the code The code is as follows:

var obj = {status:0, msg:'xxxx'};
var data = obj.status || 'xxxx';

This is always "hidden" "Yes!"

I emphasize again: the implicit conversion of value 0 into a Boolean value of false is determined by Boolean decisions in if(), while(), and between the two semicolons of for(;;) Conditional expression.