Randomly generate spelling mistake in string

Question

My code will generate a spelling mistake inside of a string 50% of the time.

It will retrieve a letter from a random index in the string like, "t" and then duplicate that letter like, "tt" and store it in the spelling mistake variable.

It will then replace, "t" with "tt" in the string to replicate a spelling mistake. (50% of the time.)

How can I improve my code to make it complete the same task, but with less lines of code and using the least amount of resources possible?

(function() {
 function replaceStr(str, pos, value) {
 var arr = str.split('');
 arr[pos] = value;
 return arr.join('');
 }
 var myString = "Stack Overflow";
 var letterIndex = Math.floor(Math.random() * myString.length); // Example: 1
 var letter = myString.charAt(letterIndex); // Example: "t"
 var mistake = letter + letter; // Example: "tt"
 // 0 -> 9 (coin toss)
 if (Math.floor(Math.random() * 10) >= 5) {
 return replaceStr(myString, letterIndex, mistake);
 } else {
 return myString;
 }
})();

Result from 10 runs:

Stack Overflow
Stackk Overflow
Stack Overflow
Sttack Overflow
Stack Overflow
Stack Oveerflow
Stack Overrflow
Stack Overfllow
Stack Overflow
Stack Oveerflow

• Joshua Taylor: Does it matter that duplicating a letter doesn't always generate a misspelled word? E.g., duplicating t in later produces latter which is another correctly spelled word.
• JamesFaix: I've never considered the potential applications for a typo machine. Interesting.
• Pharap: Sometimes less lines of code and less resources are opposite things.
• user15331: But, why would you need to make a spell error? Just curious :-)
• User1000547: @xyz reassons...

Answer 1

Break it down into one function that randomly repeats a character, and another function which randomly executes that function 50% of the time:

function repeatRandomChar(str) {
 var i = Math.floor(Math.random() * str.length);
 return str.slice(0, i+1) + str.slice(i);
}
function randomTypo(str) {
 return Math.random() > 0.5 ? repeatRandomChar(str) : str;
}

Now you can just call randomTypo('Stack Overflow') 10 times to reproduce your sample data.

Note also that the original code has needless multiplication, in that

 Math.random() > 0.5 

is equivalent to:

 (Math.random() * 10) >= 5

• 200_success: Your elimination of the needless multiplication with Math.random() < 0.5 is also worth pointing out.
• Michael Urman: Wouldn't it be more efficient to avoid an extra concatenation by replacing str.slice(0, i+1) + str.charAt(i) + str.slice(i+1) with str.slice(0, i+1) + str.slice(i)?
• Jonah: @MichaelUrman Love it. I made the change.
• Stack Exchange Broke The Law: Actually Math.random() >= 0.5 would be equivalent to (Math.random() * 10) >= 5. In this case it doesn't matter whether you check >= or <, but for any other number (say 7 and 0.7) it does.
• DarioP: One may also want to check for whites.

Answer 2

This code is at the end of your function:

// 0 -> 9 (coin toss)
if (Math.floor(Math.random() * 10) >= 5) {
 return replaceStr(myString, letterIndex, mistake);
} else {
 return myString;
}

Half of the time on average, all the code above this code will not matter, because you simply return myString unchanged. So it would be better to move this condition higher, before the other logic, such as picking a random position to introduce the mistake, etc.

And instead of splitting the string and changing a position, perhaps it's more efficient to work with substrings.

Something like this:

function addMistake(myString) {
 // 0 -> 9 (coin toss)
 if (Math.floor(Math.random() * 10) < 5) {
 return myString;
 }
 function replaceStr(str, pos, value) {
 return str.substr(0, pos) + value + str.substr(pos + 1);
 }
 var letterIndex = Math.floor(Math.random() * myString.length); // Example: 1
 var letter = myString.charAt(letterIndex); // Example: "t"
 var mistake = letter + letter; // Example: "tt"
 return replaceStr(myString, letterIndex, mistake);
}

Answer 3

You can reduce your code, merely avoiding useless complications, like this:
(also improved using @Michael Urman's idea to compact the substitution statement)

function doMistake(str) {
 if (Math.floor(Math.random() * 10) >= 5) {
 var letterIndex = Math.floor(Math.random() * str.length);
 str = str.substr(0, letterIndex + 1) + str.substr(letterIndex);
 }
 return str;
}

Here it is working with the example provided by the OP:

function doMistake(str) {
 if (Math.floor(Math.random() * 10) >= 5) {
 var letterIndex = Math.floor(Math.random() * str.length);
 str = str.substr(0, letterIndex + 1) + str.substr(letterIndex);
 }
 return str;
}
for (var i = 0; i < 10; i++) {
 console.log(doMistake('Stack Overflow'));
}

• Michael Urman: Wouldn't it be more efficient to avoid an extra concatenation by replacing str.substr(0, letterIndex) + str.charAt(letterIndex) + str.substr(letterIndex) with str.substr(0, letterIndex + 1) + str.substr(letterIndex)?
• cFreed: @MichaelUrman Wow, nice improvement, and a clever one! I edit my answer with it. Thanks!

Answer 4

One could String.replace() with an replacement-function. Moreover I have incorporated a few of the suggestions already made.

var myString = "Stack Overflow";
var letterIndex; // Example: 1
var letter; // Example: "t"
function replacer(match, offset, string) {
 if (offset === letterIndex) {
 return match + match;
 }
}
// 0 -> 9 (coin toss)
if (Math.random() >= 0.5) {
 letterIndex = Math.floor(Math.random() * myString.length);
 letter = myString.charAt(letterIndex);
 return myString.replace(letter, replacer);
} else {
 return myString;
}