# 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
}]

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.

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

## Example

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));

## Output

The output in the console will be −

67.5