Insert a string at a specific index

Beefing up the string manipulation toolkit

Operating on strings can be like herding cats—frustrating. Good news, though. You can simply extend the String.prototype to include a custom splice method:

String.prototype.splice = function(idx, rem, str) {
    return this.slice(0, idx) + str + this.slice(idx + Math.abs(rem)); // Slice, Dice, and Add Spice!
};
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Magically, you can now just simply do:

var myString = "Hello World";
myString = myString.splice(5, 0, " new");  // "Hello new World" - now *that's* a new world!
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This method ensures you keep your characters safe by utilizing good ol' absolute values.

Warming up to negative indexes

To cater for those brave souls venturing into negative indexes, the splice function can be enhanced to work like a charm:

String.prototype.splice = function(idx, rem, str) {
    if (idx < 0) {
        idx = this.length + idx; // Negative, Ghost Rider. The pattern is full.
        if (idx < 0) {
            idx = 0; // Don't go raising the dead now.
        }
    }
    return this.slice(0, idx) + str + this.slice(idx + Math.abs(rem)); // Cut, paste and voila!
};
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Negative indexes are now considered, and a string can be inserted with respect to the end of the target string. How cool is that!

Impeccable slicing

While removal might not be on your agenda and you favor simplicity, this version of splice might just do the trick:

String.prototype.splice = function(idx, str) {
    return this.slice(0, idx) + str + this.slice(idx); // It's like adding a cherry on top of the sundae.
};
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Opt for this if you strictly need to insert, not remove, certain characters.

Understanding string awesomeness: slice vs substring

Slice to the chase

The slice() method can accept the notorious negative indexes, making it a versatile comrade in your code.

Safeguard with substring

If you're in a twist with your indexes, substring() gives a soft landing; if the second parameter dares to be smaller, it swaps the arguments gracefully.

Index-based insertions

The String.prototype.insert function is an excellent aid for string insertion using indexes alone:

String.prototype.insert = function(index, string) {
  return this.substring(0, index) + string + this.substring(index); // Skilful surgery on text.
};
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Code reusability and modularity

Streamlining with prototype methods

By adding methods to the String.prototype, we create a treasure trove of functions, fostering code reusability that stretches across galaxies.

The beauty of negative indexing

Negative indexing support for string manipulation is a key to unlock more flexible and powerful functions, paralleling JavaScript's native behaviours (say, the Array.prototype.slice).

Code simplicity and clarity

Always strive to create simpler and more intuitive implementations, ensuring your code reads like a book and maintenance is an evening breeze.

Real-world example

Imagine if we were to insert "bar " into the string "fooqux" at index 4. How charming it is to apply our prototype enhancement:

"fooqux".splice(4, 0, "bar "); // Returns "foobar qux" - And just like that, we're at the bar!
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Just a kind reminder for the gentle coder: When extending built-in prototypes, tread lightly to avoid stomping on other libraries and breaking future JavaScript updates.