Nullish Coalescing Operator (??)

The nullish coalescing operator (??) provides a way to define a fallback value when dealing with null values. It returns the right-hand operand when the left-hand operand is null, otherwise it returns the left-hand operand.

Syntax

leftExpr ?? rightExpr

Description

The ?? operator is particularly useful when you want to provide default values for potentially null variables or expressions. Unlike logical OR (||), which returns the right-hand operand for any falsy value (including 0, "", false), the nullish coalescing operator only does this for null values.

This makes it ideal for cases where 0, empty strings, or false are valid values that should be preserved.

Examples

Basic Usage

// With variables that might be null
username = null
defaultName = "Guest"

displayName = username ?? defaultName
log displayName  // Outputs: "Guest"

// With non-null values
username = "Alice"
displayName = username ?? defaultName
log displayName  // Outputs: "Alice"

Comparison with Logical OR

// Nullish coalescing preserves falsy values that aren't null
score = 0
defaultScore = 100

// Using ??
finalScore = score ?? defaultScore
log finalScore  // Outputs: 0 (preserves the 0)

// Using ||
finalScore = score || defaultScore
log finalScore  // Outputs: 100 (treats 0 as falsy)

Working with Objects

// Accessing potentially missing object properties
user = {
  name: "Alice",
  age: 30
}

// If bio doesn't exist, use a default
bio = user.bio ?? "No bio available"
log bio  // Outputs: "No bio available"

Working with Database Values

// Getting values from a database with fallbacks
highScore = localDbGet("highscore") ?? 0
log highScore  // Outputs: 0 if the value doesn't exist

// In conditionals
if score > (localDbGet("highscore") ?? 0) (
  localDbSet("highscore", score)
  log "New high score!"
)

Chaining Nullish Coalescing

You can chain the ?? operator to try multiple fallback values:

// Try multiple sources for a value
value = primarySource ?? backupSource ?? defaultValue

// Example with user preferences
fontSize = userPrefs.fontSize ?? siteDefaults.fontSize ?? 16

Nullish Assignment (??=)

OSL also supports the nullish assignment operator (??=), which assigns the right-hand value only if the left-hand value is null:

// Only assigns if the variable is null
variable ??= 10

// Equivalent to:
if variable == null (
  variable = 10
)

Notes

• The ?? operator only considers null as the trigger for using the fallback value

• Empty strings, 0, and false are all considered valid values and will not trigger the fallback

• The ?? operator has relatively low precedence, but explicit parentheses can be used for clarity

• The nullish coalescing operator is particularly useful when working with database values, API responses, or optional object properties