Warning Cannot Read Property foreach of Undefined Use force to Continue

If you've ever used JavaScript's split method, there's a good chance that you've encountered the following error: TypeError: Cannot read property 'split' of undefined.

There are a few reasons why you would receive this error. Most likely it's just a basic misunderstanding of how split works and how to iterate through arrays.

For example, if you try to submit the following code for the Find the Longest Word in a String challenge:

                function findLongestWord(str) {    for(let i = 0; i < str.length; i++) {     const array = str.split(" ");     array[i].split("");   } }  findLongestWord("The quick brown fox jumped over the lazy dog");              

it will throw the TypeError: Cannot read property 'split' of undefined error.

The split method

When split is called on a string, it splits the string into substrings based on the separator passed in as an argument. If an empty string is passed as an argument, split treats each character as a substring. It then returns an array containing the substrings:

                const testStr1 = "Test test 1 2"; const testStr2 = "cupcake pancake"; const testStr3 = "First,Second,Third";  testStr1.split(" "); // [ 'Test', 'test', '1', '2' ] testStr2.split(""); // [ 'c', 'u', 'p', 'c', 'a', 'k', 'e', ' ', 'p', 'a', 'n', 'c', 'a', 'k', 'e' ] testStr3.split(","); // [ 'First', 'Second', 'Third' ]                              

Check out MDN for more details about split.

The problem explained with examples

Knowing what the split method returns and how many substrings you can expect is the key to solving this challenge.

Let's take another look at the code above and see why it's not working as expected:

                function findLongestWord(str) {    for(let i = 0; i < str.length; i++) {     const array = str.split(" ");     array[i].split("");   } }  findLongestWord("The quick brown fox jumped over the lazy dog");                              

Splitting str into an array like this (const array = str.split(" ");) works as expected and returns [ 'The',   'quick',   'brown',   'fox',   'jumped',   'over',   'the',   'lazy',   'dog' ].

But take a closer look at the for loop. Rather than using the length of array as a condition to iterate i, str.length is used instead.

str is "The quick brown fox jumped over the lazy dog", and if you log str.length to the console, you'll get 44.

The last statement in the body of the for loop is what's causing the error: array[i].split("");. The length of array is 9, so i would quickly go way over the maximum length of array:

                function findLongestWord(str) {    for(let i = 0; i < str.length; i++) {     const array = str.split(" ");     console.log(array[i]);     // array[0]: "The"     // array[1]: "quick"     // array[2]: "brown"     // ...     // array[9]: "dog"     // array[10]: undefined     // array[11]: undefined   } }  findLongestWord("The quick brown fox jumped over the lazy dog");                              

Calling array[i].split(""); to split each string into substrings of characters is a valid approach, but it will throw TypeError: Cannot read property 'split' of undefined when it's passed undefined.

How to solve Find the Longest Word in a String with split

Let's quickly go over some pseudo code for how to solve this problem:

1. Split str into an array of individual words
2. Create a variable to track the greatest word length
3. Iterate through the array of words and compare the length of each word to the variable mentioned above
4. If the length of the current word is greater than the one stored in the variable, replace that value with the current word length
5. Once the length of every word is compared with the maximum word length variable, return that number from the function

First, split str into an array of individual words:

                function findLongestWordLength(str) {   const array = str.split(" "); }              

Create a variable to keep track of the longest word length and set it to zero:

                function findLongestWordLength(str) {   const array = str.split(" ");   let maxWordLength = 0; }              

Now that the value of array is ['The', 'quick', 'brown', 'fox', 'jumped', 'over', 'the', 'lazy', 'dog'], you can use array.length in your for loop:

                function findLongestWordLength(str) {   const array = str.split(" ");   let maxWordLength = 0;      for (let i = 0; i < array.length; i++) {        } }              

Iterate through the array of words and check the length of each word. Remember that strings also have a length method you can call to easily get the length of a string:

                function findLongestWordLength(str) {   const array = str.split(" ");   let maxLength = 0;      for (let i = 0; i < array.length; i++) {     array[i].length;   } }              

Use an if statement check if the length of the current word (array[i].length) is greater than maxLength. If so, replace the value of maxLength with array[i].length:

                function findLongestWordLength(str) {   const array = str.split(" ");   let maxLength = 0;      for (let i = 0; i < array.length; i++) {     if (array[i].length > maxLength) {       maxLength = array[i].length;     }   } }              

Finally, return maxLength at the end of the function, after the for loop:

                function findLongestWordLength(str) {   const array = str.split(" ");   let maxLength = 0;      for (let i = 0; i < array.length; i++) {     if (array[i].length > maxLength) {       maxLength = array[i].length;     }   }        return maxLength; }              

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Source: https://www.freecodecamp.org/news/cannot-read-property-split-of-undefined-error/

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