Simple function to repeat a call until a certain condition is met.

repeater.js

// like repeatUntil but defers actual execution until explicitly called.
// Allows the use of different until conditions, or none at all. (repeat until explicitly canceled.)

function repeater({ interval, repeat }) {
  let isCanceled = false;
  let until;
  
  function cancel() {
    isCanceled = true;
  }
  
  function conditionallyCancel() {
    if (until()) {
      cancel();
    }
  }

  function repeatUntil(nextUntil) {
    // If called with a new until condition, use the new one.
    until = nextUntil;
    
    // Swallow double calls.
    if (!isCanceled) return;
    
    isCanceled = false;
    
    function callRepetition() {
      // NOTE: Uncommenting this would result in 0 calls if until() immediately evaluates to true.
      // conditionallyCancel();
      // 
      // if (isCanceled) return;
      
      const callTime = Date.now();
      
      return Promise.resolve(repeat())
        .then(() => {
          conditionallyCancel();
          
          if (isCanceled) return;

          const delta = Date.now() - callTime;

          // If we took longer, then repeat immediately.
          if (delta >= interval) {
            callRepetition();
            return;
          }

          // Otherwise, wait the remaining time.
          setTimeout(() => {
            conditionallyCancel();
            
            if (isCanceled) return;
            
            callRepetition();
          }, interval - delta);
        })
        // Swallowing errors.
        .catch((error) => {
          cancel();
          console.error('Error in repeateUntil:', error);
        })
      ;
    }

    callRepetition();
  }

  function repeatUntilCanceled() {
    repeatUntil(() => false);
  }
  
  return {
    cancel,
    repeat: repeatUntilCanceled,
    repeatUntil,
  };
}

repeatUntil.js

function repeatUntil({ interval, repeat, until }) {
  // let callTime;
  let isCanceled = false;

  function cancel() {
    isCanceled = true;
  }

  function conditionallyCancel() {
    if (until()) {
      cancel();
    }
  }

  function callRepetition() {
    // NOTE: Uncommenting this would result in 0 calls if until() immediately evaluates to true.
    // conditionallyCancel();
    // 
    // if (isCanceled) return;
    
    const callTime = Date.now();
    
    return Promise.resolve(repeat())
      .then(() => {
        conditionallyCancel();
        
        if (isCanceled) return;

        const delta = Date.now() - callTime;

        // If we took longer, then repeat immediately.
        if (delta >= interval) {
          callRepetition();
          return;
        }

        // Otherwise, wait the remaining time.
        setTimeout(() => {
          conditionallyCancel();
          
          if (isCanceled) return;
          
          callRepetition();
        }, interval - delta);
      })
      // Swallowing errors.
      .catch((error) => {
        cancel();
        console.error('Error in repeateUntil:', error);
      })
    ;
  }

  callRepetition();

  return {
    cancel,
  };
}

module.exports = repeatUntil;