Guard against double resolve/reject in wrappers

Question

When adapting callback APIs to promises, demonstrate how to prevent accidental multiple settle calls.

Accepted Answer

function fromCallback(register) {
  return new Promise((resolve, reject) => {
    let settled = false

const onceResolve = (value) => {
      if (settled) return
      settled = true
      resolve(value)
    }

const onceReject = (err) => {
      if (settled) return
      settled = true
      reject(err)
    }

try {
      register(onceResolve, onceReject)
    } catch (err) {
      onceReject(err)
    }
  })
}

// Usage: adapt odd callback API
const p = fromCallback((ok, fail) => {
  ok('first')
  ok('second') // ignored safely
})
p.then(console.log) // first

Guard against double resolve/reject in wrappers

Answer