How to find a word in the string

Question

I want to find a word in the string and replace it with new one. If the first letter in the word is upper case, the function should replace it with upper case, for example if you mean to replace the word "Book" with the word "dog", it should be replaced as "Dog".

Is there a better solution than this:

function myReplace(str, before, after) {
 var arr = str.split(" ");
 if(arr.indexOf(before) > -1){
 if(/[a-z]/.test(before[0])){
 arr.splice(arr.indexOf(before), 1, after); 
 }else if(/[A-Z]/.test(before[0])){
 arr.splice(arr.indexOf(before), 1, after[0].toUpperCase() + after.slice(1)); 
 }
 }
 return arr.join(" ");
} 
myReplace("He is Sleeping on the couch", "Sleeping", "sitting");
//Should return "He is Sitting on the couch"
myReplace("Let us go to the store", "store", "mall");
//Should return "Let us go to the mall"
myReplace("His name is Tom", "Tom", "john");
//Should return "His name is John"

• Mast: Define 'better'.
• Matt: The edit is too small for me to do but I recommend removing the > at the beginning of this post as your text is not a quote.

Answer 1

It doesn't look great to split words, mutate the array, then join back. It might be better (and simpler) using a regex.

The logic of matching the case of the starting letter of the replacement has some problems:

• Instead of applying the case transformation for each match, you could apply it once to the after parameter
• The replacement is not symmetric: if before starts with uppercase, you transform after to start with uppercase, but you don't do it the other way around, despite saying "vice versa" in your description.
• It would be good to clarify what should happen if before or after are mixed case, for example sLEEping -> SiTTIng

With the above suggestions applied, a simpler, cleaner and more efficient implementation:

function myReplace(str, before, after) {
 if (/[A-Z]/.test(before)) {
 after = after[0].toUpperCase() + after.slice(1);
 }
 return str.replace(new RegExp(before, 'g'), after);
}