FE Interview Test Questions - Approach

Question 1

Implement

Promise.all

Implement the Promise.all() function that resolves to an array of result

Answer 1

• return a new promise
  
  • collect the results in an array - based on the index [p1, p2, p3] - [r1, r2, r3]
  • track if the promises have (un)resolved
  • resolve with empty array if iterable is empty
  • loop over each of the individual promises
    
    • kick off the promise and await the result
    • assign the result back to the results
    • update the state which tracks if the promises have resolved
    • check if all promises have resolved - if so, then resolve the “new” promise
    • reject the “new” promise if error

Question 2

Implement

flatten

Implement a function flatten that returns a newly-created array with all sub-array elements concatenated recursively into a single level.

Answer 2

• loop through array
• if item is array then spread / flatten (recursive) and add to return array
• else add item to return array
• return acc.concat(Array.isArray(item) ? flatten(item) : item)

• this is recursion, we could do it iteratively

• create a results array
• loop over the array to flatten - clone first
  
  • take first element - shift to remove the element from the clone
  • check if it is array
  • if so, spread the array and add to the start of the cloned array for further checks - unshift
  • else, it’s not an array so push it to the results array

Question 3

Implement

filter

Implement Array.prototype.filter.

Answer 3

• loop over the array (this)
• for each element we want to call the callback func
  
  • if true, add element to results arr
  • else don’t
• reduce vs for loop
• what if the callback func modifes the value? - cache it first