javascript - What is this hr = hr>=12 ? hr-12 : hr; means? - Stack Overflow

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I am practicing this Clock tutorial. Everything in the code is clear to me. Accept this hr = hr>=12 ? hr-12 : hr; is not clear me. May be it's a if else statement.

Can any explain what this code is doing?

Thanks :)

I am practicing this https://developer.mozilla/en/Canvas_tutorial/Basic_animations Clock tutorial. Everything in the code is clear to me. Accept this hr = hr>=12 ? hr-12 : hr; is not clear me. May be it's a if else statement.

Can any explain what this code is doing?

Thanks :)

• javascript

Share Improve this question Follow edited Jan 16, 2020 at 6:20 sandeep asked Jun 15, 2012 at 6:30 sandeepsandeep 92.9k2424 gold badges140140 silver badges156156 bronze badges

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8 Answers 8

Sorted by: Reset to default 4

This is the ternary operator (?:) Here is the simple explanation of what is being done here:

if(hr>=12)
{
    hr=hr-12;
}
//or else hr will have its same value

if (hr >= 12) {
   hr = hr - 12;
} 

if hr does not meet that criteria hr should effectively be left untouched.

its called Ternary operation

It means

if(hr>=12)
 hr=hr-12;
else
 hr=hr;

the following is enough

if(hr>=12)
     hr=hr-12;

It's a ternary operator, of the form:

condition ? if_true | if_false 

If you add more brackets for readibility, it can bee:

hr = ( (hr >= 12) ? (hr - 12) : hr )

That is, if more than 12, subtract 12, and store back to hr.

hr= hr>=12 ? hr-12 : hr; is same to if( hr >= 12 ) hr = hr-12 else hr = hr;

the bool ? expr_a : expr_b is a operator that when bool is true, expr_a is evaluated and its value will be used as the whole expr's value, otherwise the expr_b will be.

It means:

if(hr>=12)
{
  hr = hr - 12;
}

Generally:

x= condition ? y : z

if condition is true, then x = y, else x = z

It has same effect as this:

hr %= 12;   //equivalent to -> hr = hr>=12 ? hr-12 : hr;

It's called a ternary operator.

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