Remove duplicates from arrays

Problem

You want to remove duplicates from arrays.

Ingredients

• two for loops
• the concat method
• the splice method

Directions

1. Given: an array with duplicate values.

   let array = ['John', 'James', 'John', 'Jane', 'John'];
   let result = removeDuplicates(array);
   console.log(result);
   // expected content of array3: ['John', 'James', 'Jane']

2. Create the removeDuplicates function and inside create a copy of the array parameter (e.g., by using the concat() method).

   function removeDuplicates(array) {
     let result = array.concat();
     ...
     return result;
   }

3. Iterate over each element in the copy.

   function removeDuplicates(array) {
     let result = array.concat();
     for(let i=0; i<result.length; ++i) {
       ...
     }
     return result;
   }

4. For each element iterate over all the other elements.

   function removeDuplicates(array) {
     let result = array.concat();
     for(let i=0; i<result.length; ++i) {
       for(let j=i+1; j<result.length; ++j) {
         ...
       }
     }
     return result;
   }

5. Compare the element with each other element.

   function removeDuplicates(array) {
     let result = array.concat();
     for(let i=0; i<result.length; ++i) {
       for(let j=i+1; j<result.length; ++j) {
         if(result[i] === result[j]) {
           ...
         }
       }
     }
     return result;
   }

6. If both elements are equal (===) then delete the duplicate using the splice() method.

   function removeDuplicates(array) {
     let result = array.concat();
     for(let i=0; i<result.length; ++i) {
       for(let j=i+1; j<result.length; ++j) {
         if(result[i] === result[j]) {
           result.splice(j--, 1);
         }
       }
     }
     return result;
   }

7. Voilá, there you got a function which removes duplicates from an array.

   let array = ['John', 'James', 'John', 'Jane', 'John'];
   let result = removeDuplicates(array);
   console.log(result); // ['John', 'James', 'Jane']

Notes

• This recipe only works for strict equality (===), not for logical equality (e.g., it doesn’t work for different objects containing the same information).
• This recipe has O(n²) runtime.

Alternative recipes

• Since ES2015: use the Set datatype which doesn’t allow duplicates by default.

  function removeDuplicates(...array) {
    return [...new Set([].concat(...array))];
  }