Typescript 4.0: Short-Circuiting Assignment Operators

append.ts

// Then
const append = (values: string[], value: string) => {
  (values ?? (values = [])).push(value)

  return values
}

// Now
const append = (values: string[], value: string) => {
  (values ??= []).push(value)

  return values
}
   
source: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-0.html#short-circuiting-assignment-operators

assign.ts

// Then
const assign = <T>(a: T, b: T) => {
  if (!a) {
    a = b
  }

  return a
}

// Now
const assign = <T>(a: T, b: T) => {
  a ||= b

  return a
}
  
source: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-0.html#short-circuiting-assignment-operators

assign2.ts

// Then
export const assign2 = (obj: { prop: string }, foo: () => string) => {
  if (!obj.prop) {
    obj.prop = foo()
  }

  return obj
}

// Now
export const assign2 = (obj: { prop: string }, foo: () => string) => {
  obj.prop ||= foo()

  return obj
}

source: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-0.html#short-circuiting-assignment-operators