Hidden power of || and &&

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) { 
  doSomething()
}

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
}

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 }

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
}

Using the && operator we can check for property presence in an object.

let name = response && response.data && response.data.name

// Same to

let name;
if (response) {
  if (response.data) {
    if (response.data.name) {
      name = response.data.name
    }
  }
}

So if response or response.data 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 && response.data && response.data.name || 'UNKNOWN'

Btw in newer versions of TypeScript, you can use a nullish coalescing, which simplifies && chains even more.

let name = response?.data?.name || 'UNKOWN'

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

References
MDN/||
MDN/&&

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Hidden power of || and &&

Nick K

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