Searching Strings, Part 3

Let's look at a few special cases when using indexOf.

Not Found

What if the substring isn't found? Let's find out. Add this code to the editor and run it.

let index_z = str.indexOf("z");
console.log("Index of 'z': ", index_z);

Try finding the index of another string not in str. Do you always get the same result when the substring cannot be found?

Duplicated Substrings

Add this code to the editor, but before you run it, guess what will happen.

let ooString = "good books put even fools in good moods";
let index_of_oo = ooString.indexOf("oo");
console.log("Index of 'oo': ", index_of_oo);

No matter how many times you run the code, index_of_oo will always be the same, even though oo appears five times.

Add this code:

let last_index_of_oo = ooString.lastIndexOf("oo");
console.log("Last index of 'oo': ", last_index_of_oo);

lastIndexOf works like indexOf, but instead of finding the first occurrence it will find the index of the last occurrence.

What about all the oo occurrences in between the first and last? You don't have to start searching from the beginning of the string. For example, if I want to find the second instance, I can start searching from after the first occurrence, like so:

let index_of_second_oo = ooString.indexOf("oo", 3);
console.log("Index of second 'oo': ", index_of_second_oo);

The second argument (3 in this example) tells indexOf what index to start searching from. I can use the use the index of the second occurrence to get the third and the third to get the fourth:

let index_of_third_oo = ooString.indexOf("oo", index_of_second_oo + 1);
let index_of_fourth_oo = ooString.indexOf("oo", index_of_third_oo + 1);
console.log("Index of third 'oo': ", index_of_third_oo);
console.log("Index of fourth 'oo': ", index_of_fourth_oo);

Tests

Now try to pass the tests described at the bottom of the starter code.