Shorthand if/else statement Javascript

问题

I'm wondering if there's a shorter way to write this:

var x = 1;
if(y != undefined) x = y;

I initially tried x = y || 1, but that didn't work. What's the correct way to go about this?

回答1:

var x = y !== undefined ? y : 1;

Note that var x = y || 1; would assign 1 for any case where y is falsy (e.g. false, 0, ""), which may be why it "didn't work" for you. Also, if y is a global variable, if it's truly not defined you may run into an error unless you access it as window.y.

---

As vol7ron suggests in the comments, you can also use typeof to avoid the need to refer to global vars as window.<name>:

var x = typeof y != "undefined" ? y : 1;

回答2:

Another way to write it shortly

bePlanVar = !!((bePlanVar == false));

// is equivalent to

bePlanVar = (bePlanVar == false) ? true : false;

// and 

if (bePlanVar == false) {
    bePlanVar = true;
} else {
    bePlanVar = false;
}

回答3:

y = (y != undefined) ? y : x;

The parenthesis are not necessary, I just add them because I think it's easier to read this way.

回答4:

Other way is using short-circuit:

x = (typeof y !== 'undefined') && y || 1

Although I myself think that ternary is more readable.

回答5:

Here is a way to do it that works, but may not be best practise for any language really:

var x,y;
x='something';
y=1;
undefined === y || (x = y);

alternatively

undefined !== y && (x = y);

来源：https://stackoverflow.com/questions/9864634/shorthand-if-else-statement-javascript