JavaScript syntax

In another question answered here I found the following JavaScript code:

function _dom_trackActiveElement(evt) {
    if (evt && evt.target) { 
        document.activeEleme开发者_如何转开发nt = evt.target == document ? null : evt.target;
    }
}

But this syntax is unknown to me, could someone explain exactly what

document.activeElement = evt.target == document ? null : evt.target;

does?

? : is the conditional operator, sometimes called the "ternary operator". As an example, a ? b : c will return b if a is true, and c otherwise.

Your code will assign null to document.activeElement if evt.target == document. Otherwise, evt.target will be assigned.

This is Ternary Operator, here is more info about it.

Its prototype is like this:

 (expression) ? true : false

Example:

 (myvar == 10) ? document.write('yes it is equal to 10') : document.write('no it is not equal to 10')

Your condition can be re-written like this too which is essentially the same:

if (evt.target == document)
{
  document.activeElement = null;
}
else
{
  document.activeElement = evt.target;
}

Hope this if-else condition makes it easier for you to understand what Ternary operator is all about. Thanks :)

It's a ternary operator, similar to that in java. Its saying that if evt.target==document then activeElement is null, otherwise activeElement is evt.target.

It is the ternary operator. It might be easier if I parenthesise it for you:

document.activeElement = ((evt.target == document) ? null : evt.target);

The basic format is:

something_boolean ? if_true_return_this : else_return_this;

So you can either use it like an if-else statement, or for assignment.

That syntax is the conditional operator (also known as ternary operator). The expression

expr1 ? expr2 : expr3

evaluates to the evaluated value of expr2 if the evaluated value of expr1 is true or to the evaluated value of expr3 otherwise.

So in your example

evt.target == document ? null : evt.target

will evaluate to null if evt.target == document is true or to evt.target otherwise. That means null is assigned to document.activeElement if evt.target == document is true and otherwise evt.target is assigned to document.activeElement:

if (evt.target == document) {
    document.activeElement = null;
} else {
    document.activeElement = evt.target;
}

This is the conditional operator, which wikipeida explains as

condition ? value if true : value if false

It is easier to read with brackets, as it makes the order of evaluation clearer

document.activeElement = (evt.target == document ? null : evt.target);

The above line can be expanded expanded to

if ( evt.target == document ){
    document.activeElement = null;
}else{
    document.activeElement = evt.target;
}

In other words, it assigns the result of the conditional expression to document.activeElement.