How to write a ternary operator (aka if) expressio

Question

For example, something like this:

var value = someArray.indexOf(3) !== -1 ? someArray.indexOf(3) : 0

Is there a better way to write that? Again, I am not seeking an answer to the exact question above, just an example of when you might have repeated operands in ternary operator expressions...

Answer 1

I think the || operator can be tailored to indexOf:

var value = ((someArray.indexOf(3) + 1) || 1) - 1;

The returned value is shifted up by 1, making 0 from -1, which is falsey and therefore gets replaced by the second 1. Then it is shifted back.

However, please keep in mind that readability is superior to avoiding repetition.

Answer 2

Not really, just use another variable.

Your example generalizes to something like this.

var x = predicate(f()) ? f() : default;

You're testing a computed value, then assigning that value to a variable if it passes some predicate. The way to avoid re-calculating the computed value is obvious: use a variable to store the result.

var computed = f();
var x = predicate(computed) ? computed : default;

I get what you mean - it seems like there ought to be some way to do this that looks a little cleaner. But I think that's the best way (idiomatically) to do this. If you were repeating this pattern a lot in your code for some reason, you might write a little helper function:

var setif = (value, predicate, default) => predicate(value) ? value : default;
var x = setif(someArray.indexOf(3), x => x !== -1, 0)

Answer 3

I personally prefer two variants:

1. Pure if, like @slebetman suggested

2. Separate function, which replaces invalid value with default one, like in this example:

function maskNegative(v, def) {
  return v >= 0 ? v : def;
}

Array.prototype.indexOfOrDefault = function(v, def) {
  return maskNegative(this.indexOf(v), def);
}

var someArray = [1, 2];
console.log(someArray.indexOfOrDefault(2, 0)); // index is 1
console.log(someArray.indexOfOrDefault(3, 0)); // default 0 returned
console.log(someArray.indexOfOrDefault(3, 123)); // default 123 returned

Answer 4

A ternary is like an if-else, if you don't need the else part, why not just a single if instead..

if ((value = someArray.indexOf(3)) < 0) value = 0;

Answer 5

For numbers

You can use the Math.max() function.

var value = Math.max( someArray.indexOf('y'), 0 );

It will keep the boundaries of the result from 0 until the first result greater than 0 if that's the case. And if the result from indexOf is -1 it will return 0 as is greater than -1.

For booleans and boolean-y values

For JS there is no general rule AFAIK specially because how falsy values are evaluated.

But if something can help you most of the time is the or operator (||):

// Instead of
var variable = this_one === true ? this_one : or_this_one;
// you can use
var variable = this_one || or_this_one;

You have to be very careful with this, because in your first example, indexOf can return 0 and if you evaluate 0 || -1 it will return -1 because 0 is a falsy value.

Answer 6

You're probably looking for a coalescing operator. Luckily, we can leverage the Array prototype to create one:

Array.prototype.coalesce = function() {
    for (var i = 0; i < this.length; i++) {
        if (this[i] != false && this[i] != null) return this[i];
    }
}

[null, false, 0, 5, 'test'].coalesce(); // returns 5

This could be further generalized to your case, by adding a parameter to the function:

Array.prototype.coalesce = function(valid) {
    if (typeof valid !== 'function') {
        valid = function(a) {
            return a != false && a != null;
        }
    }

    for (var i = 0; i < this.length; i++) {
        if (valid(this[i])) return this[i];
    }
}

[null, false, 0, 5, 'test'].coalesce(); // still returns 5
[null, false, 0, 5, 'test'].coalesce(function(a){return a !== -1}); // returns null
[null, false, 0, 5, 'test'].coalesce(function(a){return a != null}); //returns false