DNA Pair Algorithm -- Help Understaning .forEach Behavior

1

Hi all! So I have my code working (yay!) but it appears as though it is not executing top-to-bottom as I expect, and I’m not sure why. :confused:

Here’s my solution: http://bit.ly/2jWY0Iu

In the middle I have a “sanity check” console log that breaks the two steps in half, but when it runs it is ~actually~ logging as if the .forEach section of the code below it has already run (adding the second letter pair to the sub-array), thus is giving me the final answer.

Why does this happen? Does .forEach have a special flag about how it is handled by the interpreter?
I’ve dug around on the .MDN and I’ve only found that .forEach cannot be chained.

2

Hi. Could you paste your code here instead? Your link only shows the first seven lines of your code.

3

Sorry about that!

Here’s my code directly, and a jsfiddle that should work https://jsfiddle.net/InkAnt908/4hzL7us6/1/

function pairElement(str){
var arr = [];
var tempArr = [];

//Step 1.
//Take str, push each char to a sub-array,
//Then push that sub-array into the main array & reset
for (var i = 0; i<str.length; i++){
tempArr.push(str[i]);
arr.push((tempArr));
tempArr = [];
}

//Up to this point, we ~should~ have arr = [["G"],["C"],["G"]]
console.log("Sanity check:",arr);

//Step 2.
//Loop through each subarray, add the companion letter to it
arr.forEach(function addPair(arr){
//console.log(arr[j],"is:",arr[j]);
while (arr.length <= 1){
if (arr[0] == "A"){
   arr.push("T");
  }
else if (arr[0] == "T"){
   arr.push("A");
  }
else if (arr[0] == "G"){
   arr.push("C");
  }
else if (arr[0] == "C"){
   arr.push("G");
  }
else{
 console.log("Something is broken :(");
 }
console.log("Add pair done, returning:",arr);
return arr;
}
});

console.log("DONE!",arr);
return arr;
}

pairElement("GCG");
4

It doesn’t seem to be the case. Look at this.
https://repl.it/FY57/1

I think it has something to do with the browser’s console (like, it doesn’t give you a snapshot of what the array looks like before you run .forEach, but instead gives you a “link” to the array).

1 Like
5

Oh wow, that’s interesting!

On that linked module it works as you described, but in the FCC courses, jsfiddle, and a local page with tags it gives me the original result: http://imgur.com/rcJ0w2r

It does the same thing in Firefox, as well… weird.