[javascript] How to create a stopwatch using JavaScript?

Solution by Mosh Hamedani

Creating a StopWatch function constructor.

Define 4 local variables

1. startTime
2. endTime
3. isRunning
4. duration set to 0

Next create 3 methods

1. start
2. stop
3. reset

start method

• check if isRunning is true if so throw an error that start cannot be called twice.
• set isRunning to true
• assign the current Date object to startTime.

stop method

• check if isRunning is false if so throw an error that stop cannot be called twice.
• set isRunning to false
• assign the current Date object to endTime.
• calculate the seconds by endTime and startTime Date object
• increment duration with seconds

reset method:

• reset all the local variables.

Read-only property

if you want to access the duration local variable you need to define a property using Object.defineProperty. It's useful when you want to create a read-only property.

Object.defineProperty takes 3 parameters

• the object which to define a property (in this case the current object (this))
• the name of the property

• the value of the key property.

• We want to create a Read-only property so we pass an object as a value. The object contain a get method that return the duration local variable. in this way we cannot change the property only get it.

The trick is to use Date() object to calculate the time.

Reference the code below

function StopWatch() {


let startTime,
    endTime,
    isRunning,
    duration = 0;

  this.start = function () {
    if (isRunning) throw new Error("StopWatch has already been started.");

    isRunning = true;

    startTime = new Date();
  };



this.stop = function () {
    if (!isRunning) throw new Error("StopWatch has already been stop.");

    isRunning = false;

    endTime = new Date();

    const seconds = (endTime.getTime() - startTime.getTime()) / 1000;
    duration += seconds;
  };

  this.reset = function () {
    duration = 0;
    startTime = null;
    endTime = null;
    isRunning = false;
  };

  Object.defineProperty(this, "duration", {
    get: function () {
      return duration;
    },
  });
}

const sw = new StopWatch();