arrays - Javascript filter based upon previous value - Stack Overflow

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I am trying to rewrite a function in ES6 to use filtering over an array of objects. Currently the code loops through the array and copies array members that meet a criteria to a new array which it then returns. The criteria is based upon the previous value in the array. I want to filter out all array items where the timestamp field of the object is < 4 minutes.

let final = [];
final.push(data[0]);

for (let i = 1, j = data.length; i < j; i++) {
    // if time difference is > 4 minutes add to our final array
    if (data[i].timestamp - data[i-1].timestamp > 240) {
        final.push(data[i]);
    }
}
return final;

There has got to be a better way of doing this. I thought of using an arrow function, but I dont see how I would access the timestamp of the previous array item object when iterating.

I am trying to rewrite a function in ES6 to use filtering over an array of objects. Currently the code loops through the array and copies array members that meet a criteria to a new array which it then returns. The criteria is based upon the previous value in the array. I want to filter out all array items where the timestamp field of the object is < 4 minutes.

let final = [];
final.push(data[0]);

for (let i = 1, j = data.length; i < j; i++) {
    // if time difference is > 4 minutes add to our final array
    if (data[i].timestamp - data[i-1].timestamp > 240) {
        final.push(data[i]);
    }
}
return final;

There has got to be a better way of doing this. I thought of using an arrow function, but I dont see how I would access the timestamp of the previous array item object when iterating.

• javascript
• arrays
• filter
• ecmascript-6

Share Improve this question Follow asked Oct 11, 2018 at 16:41 GregGreg 1,85333 gold badges1919 silver badges2626 bronze badges 1

• Two answers with the same solution. Thanks to both of you! – Greg Commented Oct 11, 2018 at 16:52

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2 Answers 2

Sorted by: Reset to default 7

The Array.filter() method passes the index (i) to the callback function, and you can use it to get the previous value from the array. To take the 1st item as well, I use the condition !i, which is evaluated to true when i is 0.

const data = [{ timestamp: 1 }, { timestamp: 3 }, { timestamp: 250 }, { timestamp: 1000 }];

const final = data.filter((o, i) => 
  !i || (o.timestamp - data[i-1].timestamp > 240)
);

console.log(final);

let prevTs = -Infinity;
const result = data.filter((d) => {
  const localResult = (d.timestamp - prevTs) > 240;
  prevTs = d.timestamp;
  return localResult;
});

Or you can use index arg in your filter callback:

data.filter((d, i) => {
  if (!i) {
    return true;
  }
  return (d.timestamp - data[i - 1].timestamp) > 240
});

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