If a variable is undefined set new variable to a defined one, without 'if' statement?

Question

I know an if statement can easily accomplish this. I'm just wondering if there's anything that could be done even more simply.

For example:

var a
var b='this has a value'
var c = don't use a, instead use b

What exactly has to be true about a for you to not want to "use" it? — Pointy
– Pointy, Commented Nov 13, 2019 at 0:42
It was the simplest way to ask my question. It looks like @CertainPerformance has given me what I need. For more detail, I'm instantiating an IndexedDB database that may never have data in it, or not until later. When I load my page, I want it to check if there's data and populate an element with that data. If not, set the values temporarily until there is data. — Dshiz
– Dshiz, Commented Nov 13, 2019 at 0:45

CertainPerformance · Accepted Answer · 2019-11-13 00:40:01Z

4

If a, if defined, will be truthy, just alternate:

var c = a || b;

let a1;
let b1 = 5;
let c1 = a1 || b1;
console.log(c1);


let a2 = 'truthy';
let b2 = 10;
let c2 = a2 || b2;
console.log(c2);

Otherwise, if a can be defined but falsey, use the conditional operator:

var c = a !== undefined ? a : b;

let a1 = null;
let b1 = 5;
let c1 = a1 !== undefined ? a1 : b1;
console.log(c1);

answered Nov 13, 2019 at 0:40

CertainPerformance

373k55 gold badges354 silver badges359 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Samuel Goldenbaum Over a year ago

in addition: let c = (typeof a === 'undefined') ? b : a;

CertainPerformance Over a year ago

Sure, if you're actually worried about there being an outer variable name called undefined, or about the global undefined property having been overwritten (in very ancient environments, pre-ES5), but those are very strange edge cases

Collectives™ on Stack Overflow

If a variable is undefined set new variable to a defined one, without 'if' statement?

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related