Question

172

Remove duplicates from arrays using reduce

rated 0 times [ 174] [ 2] / answers: 1 / hits: 16500 / 7 Years ago, thu, may 4, 2017, 12:00:00

I am trying to remove duplicates from a list of arrays. The way I was trying to do this is by using reduce to create an empty array that pushes all undefined indexes onto that array. I am getting errors though that

if(acc[item]===undefined){

      ^

TypeError: Cannot read property '1' of undefined

my function below:

function noDuplicates(arrays) {

  var arrayed = Array.prototype.slice.call(arguments);



  return reduce(arrayed, function(acc, cur) {

    forEach(cur, function(item) {

      if (acc[item] === undefined) {

        acc.push(item);

      }

      return acc;

    });

  }, []);

}



console.log(noDuplicates([1, 2, 2, 4], [1, 1, 4, 5, 6]));

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mahogany

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3 Years ago

answered 7 Years ago estefanib · Accepted Answer

There are a number of issues with how you're calling methods and where you return acc from:

function noDuplicates(arrays) {

  var arrayed = Array.prototype.slice.call(arguments);



  // reduce is a method of an array, so call it as a method

  // return reduce(arrayed, function(acc, cur) {

  return arrayed.reduce(function(acc, cur) {

  

    // Same with forEach

    cur.forEach(function(item) {

      if (acc[item] === undefined) {

        acc.push(item);

      }

       // Return acc from the reduce callback, forEach returns undefined always

       // return acc;

    });

    return acc;

  }, []);

}



console.log(noDuplicates([1, 2, 2, 4], [1, 1, 4, 5, 6]));

You could also call reduce directly on arguments using call:

Array.prototype.reduce.call(arguments, function(acc, curr) {

  // ...

});

The above makes your code run, but it doesn't produce the correct output as the test:

  if (acc[item] === undefined)

doesn't do what you want. What you need to do is remember each value and only push it to acc if it's not been seen before:

function noDuplicates(arrays) {

  var arrayed = Array.prototype.slice.call(arguments);

  var seen = {};



  return arrayed.reduce(function(acc, cur) {

    cur.forEach(function(item) {

      if (!seen[item]) {

        acc.push(item);

        seen[item] = true;

      }

    });

    return acc;

  }, []);

}



console.log(noDuplicates([1, 2, 2, 4], [1, 1, 4, 5, 6]));

Some other approaches:

// A more concise version of the OP

function noDupes() {

  return [].reduce.call(arguments, function(acc, arr) {

    arr.forEach(function(value) {

      if (acc.indexOf(value) == -1) acc.push(value);

    });

    return acc;

   },[]);

}

 

console.log(noDupes([1, 2, 2, 4], [1, 1, 4, 5, 6]));



// Some ECMAScript 2017 goodness

function noDupes2(...args){

 return [].concat(...args).filter((v, i, arr) => arr.indexOf(v)==i);

}



console.log(noDupes2([1, 2, 2, 4], [1, 1, 4, 5, 6]));