Name this typeError? Probably scope error

Question

WARNING: Probably not a typeError. Doing scripting algorithms for fun. Trying to learn something/sharpen problem solving => I commented out the undefined array. I can't tell why it's throwing an error.

 function arrayManipulation(n, queries) { var array = new Array(n).fill(0) var x = 0 var recurring = (argmnt, query, y) => { //problem start here var start = query[0], end = query[1] //problem end somewhere else var newArg = argmnt.map((el, index) =>{ if(index+1 >= start && index+1 <= end){ return query[2] + el }else{ return el } }) console.log(newArg) if ( y < queries.length ){ y += 1 recurring(newArg, queries[y], y) }else{ return newArg } } var solution = recurring(array, queries[x], x) } arrayManipulation(5, [[1, 2, 100], [2, 5, 100], [3, 4, 100]]) 

Although the script runs fine and give me the desired output. It throws a stderr in node or typeerror in JS. I just want to know why that is.

Answer 1

The problem is with your y variable. In your check y < queries.length you first check for the length and then increment.

If your array has a length of 3 (as it does in your example), you will check if y is less than 3 and if it's 2 , it passes the test, gets incremented to 3 and then you pass queries[3] to the next recurring() invocation. The last element of queries is queries[2] , though. So queries[3] is undefined and inside the function, you try query[0] which is accessing property 0 of undefined . That's the error.

 function arrayManipulation(n, queries) { var array = new Array(n).fill(0) var x = 0 var recurring = (argmnt, query, y) => { //problem start here var start = query[0], end = query[1] //problem end somewhere else var newArg = argmnt.map((el, index) =>{ if(index+1 >= start && index+1 <= end){ return query[2] + el }else{ return el } }) console.log(newArg) y += 1 // now is here if ( y < queries.length ){ // was here recurring(newArg, queries[y], y) }else{ return newArg } } var solution = recurring(array, queries[x], x) } arrayManipulation(5, [[1, 2, 100], [2, 5, 100], [3, 4, 100]]) 

You have two choices: either increment first and then do the check (as in the snippet above), or check for queries.length - 1 , like this:

if ( y < queries.length - 1 ){

---

Also, a bit of an optimization. You don't need the y variable. It's basically just x but passed along as an argument. You have access to x from recurring() so you might as well use it :

 function arrayManipulation(n, queries) { var array = new Array(n).fill(0) var x = 0 var recurring = (argmnt, query) => { //problem start here var start = query[0], end = query[1] //problem end somewhere else var newArg = argmnt.map((el, index) => { if (index + 1 >= start && index + 1 <= end) { return query[2] + el } else { return el } }) console.log(newArg) x += 1 if (x < queries.length) { recurring(newArg, queries[x], x) } else { return newArg } } var solution = recurring(array, queries[x]) } arrayManipulation(5, [ [1, 2, 100], [2, 5, 100], [3, 4, 100] ]) 

Answer 2

Here is what I got from your script:

{
  "message": "Uncaught TypeError: Cannot read property '0' of undefined",
  "filename": "https://stacksnippets.net/js",
  "lineno": 20,
  "colno": 22
}

This is normal because of this part:

if ( y < queries.length ){
      y += 1
      recurring(newArg, queries[y], y)

You have 3 queries. At one point, y will equal 2 .

It will be incremented and will equal 3 . queries[3] doesn't exist and will be undefined . (because, you do know that an array's first index is 0, do you?)

Then, in the next recurring , you'll call query[0] on it, but it is undefined : it will crash.

That's all ;) You if condition is not good ;)

It must be if ( y < queries.length - 1 )