How to calculate total time between a list of entries?

Let's say, we have an array that contains some data about the speed of a motor boat during upstreams and downstreams like this ?

Following is our sample array ?

const arr = [{
   direction: 'upstream',
   velocity: 45
}, {
   direction: 'downstream',
   velocity: 15
}, {
   direction: 'downstream',
   velocity: 50
}, {
   direction: 'upstream',
   velocity: 35
}, {
   direction: 'downstream',
   velocity: 25
}, {
   direction: 'upstream',
   velocity: 40
}, {
   direction: 'upstream',
   velocity: 37.5
}];

console.log(arr);

[
  { direction: 'upstream', velocity: 45 },
  { direction: 'downstream', velocity: 15 },
  { direction: 'downstream', velocity: 50 },
  { direction: 'upstream', velocity: 35 },
  { direction: 'downstream', velocity: 25 },
  { direction: 'upstream', velocity: 40 },
  { direction: 'upstream', velocity: 37.5 }
]

We are required to write a function that takes in such type of array and finds the net velocity (i.e., velocity during upstream - velocity during downstream) of the boat during the whole course.

Using reduce() Method

So, let's write a function findNetVelocity(), iterate over the objects and calculate the net velocity. The full code for this function will be ?

const arr = [{
   direction: 'upstream',
   velocity: 45
}, {
   direction: 'downstream',
   velocity: 15
}, {
   direction: 'downstream',
   velocity: 50
}, {
   direction: 'upstream',
   velocity: 35
}, {
   direction: 'downstream',
   velocity: 25
}, {
   direction: 'upstream',
   velocity: 40
}, {
   direction: 'upstream',
   velocity: 37.5
}];

const findNetVelocity = (arr) => {
   const netVelocity = arr.reduce((acc, val) => {
      const { direction, velocity } = val;
      if(direction === 'upstream'){
         return acc + velocity;
      }else{
         return acc - velocity;
      };
   }, 0);
   return netVelocity;
};

console.log(findNetVelocity(arr));

67.5

Using for...of Loop

We can also solve this using a traditional for...of loop for better readability:

const findNetVelocityLoop = (arr) => {
   let netVelocity = 0;
   
   for (const entry of arr) {
      if (entry.direction === 'upstream') {
         netVelocity += entry.velocity;
      } else {
         netVelocity -= entry.velocity;
      }
   }
   
   return netVelocity;
};

console.log(findNetVelocityLoop(arr));

67.5

How It Works

The calculation works as follows:

• Upstream velocities are added: 45 + 35 + 40 + 37.5 = 157.5
• Downstream velocities are subtracted: 15 + 50 + 25 = 90
• Net velocity = 157.5 - 90 = 67.5

Conclusion

Both approaches effectively calculate net velocity by adding upstream values and subtracting downstream values. The reduce() method provides a functional programming approach, while the for...of loop offers more explicit control flow.