Javascript Ternary operator with empty else

问题

I'm trying to convert the following if-else to it's ternary operator representation in javascript as follows

var x = 2;
if (x === 2) {alert("2");}
else
         { //do nothing}

But when I do this:

(t==2)?(alert("1")):();

Chrome throws a SyntaxError.

My question is - How to have a ternary operator in javascript with an empty "else" branch - i.e the part that comes after ":". Also, is this allowed- using a ternary operator in javascript to execute statements - NOT do assignment.

Also: the above code was just a base case. I'm actually trying to get all the DOM elements of the page as an array (called all2) and then add only those elements to another array (called only) only if they have non-null class names. Here is my code:

all2.forEach(function(e){ e.getAttribute("class") ? (only.push(e.getAttribute("class"))) : (); });

If I leave the third operand blank, it throws a syntax error. Passing a null works

回答1:

Answer to your real question in the comments:

all2.forEach(function (e) {
    e.getAttribute("class") && only.push(e.getAttribute("class"));
});

回答2:

Do this :

(t==2)?(alert("1")):null;

You could replace null by any expression that has no side effect. () is not a valid expression.

回答3:

You putted a lot of useless parentheses, and the best NULL value in js is undefined.

document.getElementById('btn-ok').onclick = function(){
  var val = document.getElementById('txt-val').value;
  
  val == 2 ? alert(val) : undefined;
}

<input id="txt-val" type="number" />
<button type="button" id="btn-ok">Ok</button>

using a single line if statement is better though

if(value === 2) alert(value);

回答4:

In that case you don't need to use Ternary operator. Ternary operator requires a third argument.

condition ? expr1 : expr2

Lokki at Conditional (ternary) Operator

You can use the if statement

if ( t == 2 ) alert(1);

回答5:

NO, you can't have empty else, better don't use the ternary operator, it requires a third argument. Go with simple if condition.

if(t==2) alert("2");

回答6:

you have a few options to do this nicely in one line:

option1 - noop function

set a global noop function:

function noop(){}
(t==2)?(alert("1")):(noop());

option2 - && operator

when you use && operater, operands are evaluted only if previos ones where true, so you could miply write:

(t==2) && alert("1");

or, for exapmle if you have an arry you want to push to, you could test it is not null before:

arr && arr.push(obj)

来源：https://stackoverflow.com/questions/31960619/javascript-ternary-operator-with-empty-else

标签

javascript

if-statement

ternary