有人可以向我解释这个javascript语句吗？

Question

I am working on a project which needs a lot of javascript coding. And I'm new to Javascript.

Just found a code but can't understand. Can somebody explain to me? I tried to search for answers but turn out with no result.

var keyCode = (e.keyCode ? e.keyCode : e.which);

I just wanna know what '?' and ':' mean in the above code. And is there any alternative way to write it?

Thanks a lot.

Answer 1

The syntax you see is called a . It is often used when an if... else... statement may be unnecessary, or too long. You will see it a lot in conditional variable assignments like the one above.

Basically, the syntax is like this:

([condition to test] ? [what should we do if true] : [what should we do if false]) .

It is shorthand for

if([condition to test]) {
    [what should we do if true];
}
else {
    [what should we do if false];
}

In that example, we are trying to assign var keyCode . Since some browsers use e.which to pass in the numeral value of the key pressed, we have to account for both e.keyCode or e.which , and assign the appropriate value.

: e.keyCode is not necessarily a boolean value. However, in JavaScript, any variable can be interpreted as boolean. e.keyCode has boolean value false if it's not defined.

Answer 2

var keyCode;
if (e.keyCode) {
    keyCode = e.keyCode;
} else {
    keyCode = e.which;
}

Answer 3

It's a shorthand for if/else.

var keyCode;
if (e.keyCode) {
    keyCode = e.keyCode;
} else {
    keyCode = e.which;
}