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Hidden power of || and &&

Nikita Kozlov
Make things happen ✨ Webdev / websec / management / drawing
Updated on ・2 min read

TLDR: Logical operators actually return one of the operands, so you can refactor code using this feature.

Usually, you may see || and && logical operators in if cases.

if (case1 || case2) { 
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So you may expect that || and && returns a boolean value, but it is only correct if values on both sides of these operators are boolean as well.

Actually, these operators return one of their arguments. E.g. if they were functions they would be implemented like this.

function or(a, b) {
  if (a) {
    return a
  return b

function and(a, b) {
  if (a) {
    return b
  return a
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It may sound and look a bit confusing, but let’s dive into examples.

let obj = { prop: 1 }

// boring part
true || false // -> true
false || true // -> true
true && false // -> false
false && true // -> false

// interesting part
obj || false // -> { prop: 1 }
false || obj  // -> { prop: 1 }

// more interesting part
true && obj // -> { prop: 1 }
obj && true // -> true 
true || obj  // -> true
obj || true // -> { prop: 1 }
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Ok, how do we use it?

Using the || operator you can set default values.

const title = props.customTitle || 'Default title'

// Same to

let title = 'Default title'
if (props.customTitle) {
  title = props.customTitle
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Using the && operator we can check for property presence in an object.

let name = response && &&

// Same to

let name;
if (response) {
  if ( {
    if ( {
      name =
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So if response or is undefined or null this assignment won’t throw any error.

Combining || and && we can get a nice construction, which checks for properties and can set a default value.

let name = response && && || 'UNKNOWN'
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Btw in newer versions of TypeScript, you can use a nullish coalescing, which simplifies && chains even more.

let name = response?.data?.name || 'UNKOWN'
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UPD: Needed to mention this can be confused with lazy evaluation, but actually it is short-circuit evaluation. Thanks to my friend Ray

UPD2: as @lukeshiru mentioned in the comments, it is better to use nullish coalescing instead of && and ||.

It's because values like 0 are treated like falsy, which can introduce bugs.

let beginnerCarDriver = {
  // ...
  yearsOfExperience: 0
let getDriveExperience = (driver) => 
  driver && driver.yearsOfExperience || "Not a driver"
console.log(getDriveExperience(beginnerCarDriver)) // Not a driver

let getDriveExperience1 = (driver) =>
  driver?.yearsOfExperience ?? "Not a driver"
console.log(getDriveExperience1(beginnerCarDriver)) // 0
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Discussion (4)

lukeshiru profile image

I love short and clean code, but I can't get behind short-circuiting, having solutions actually designed for this scenarios, like optional chaining (?.), nullish coalescing (??), and the good old ternary (condition ? ifTrue : ifFalse).
Also, short-circuiting can bite you quite easily due to falsy values like 0, "", and so on.
You mentioned this operators a little, buy your last example should be:

const name = response?.data?.name ?? "UNKOWN";
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And should be the preferred way. The code is still simple, but the result is more predictable.

Other than that, nice post covering this "loopholes" in JS logic!


patarapolw profile image
Pacharapol Withayasakpunt

Elvis cuts out not only undefined, but also null.

Still, it is much safer than short circuiting.

kozlovzxc profile image
Nikita Kozlov Author

Thanks! I've added a note to the post and mentioned you 🙏

peterlunch profile image

Nice post, short circuit logic has the added benefit of also making your code a lot cleaner.