You can simnplify your code by using list comprehension for creating 'phrase_list' and 'new_indexes'. You could also use enumerate(indexes) to loop through the indexes and insert the word at a specified location.

In the attempt I've passed all test, but it return this error:

===
TypeError: Cannot read property 'length' of null
at comp
at /home/codewarrior/index.js:34:19
at /runner/frameworks/javascript/cw-2.js:152:11
at Promise._execute
at Promise._resolveFromExecutor
at new Promise
at describe
at /home/codewarrior/index.js:16:5
at /home/codewarrior/index.js:117:5
at Object.handleError

I have tried my solution on node.js and it doesn't complain about this error.

I'm not sure it's the only problem with your code, but it returns False if both arrays are empty (the description requires to return False if they are None, but not if they are empty).