How to use a slice or substr function and what is the difference between them?

slice()

The `slice()` method takes two optional arguments:

• `startIndex`: The index where you want to begin your extraction (inclusive).
• `endIndex`: The index where you want to end your extraction (exclusive).

Syntax: 

str.slice(startIndex, endIndex)

• If `endIndex` is omitted, `slice()` extracts to the end of the string
• It also supports negative indices

Example:

let str = "Hello, World!";

console.log(str.slice(0, 5)); // "Hello"

console.log(str.slice(7)); // "World!"

console.log(str.slice(-6)); // "World!"

substr()

The `substr()` method also takes two arguments:

• `startIndex`: The index where you want to begin your extraction.
• `length`: The number of characters you want to extract (optional).

Syntax:

str.substr(startIndex, length)

• If `length` omitted, `substr()` extracts to the end of the string
• It also supports negative `startIndex`

Example:

let str = "Hello, World!";

console.log(str.substr(0, 5)); // "Hello"

console.log(str.substr(7)); // "World!"

console.log(str.substr(-6)); // "World!"

Key Differences

1. `slice()` uses start and end indices, while `substr()` uses start index and length.

2. `slice()` is more flexible and works consistently with arrays.

3. `substr()` is considered somewhat deprecated.

Conclusion

While `substr()` can be useful in certain scenarios, `slice()` is generally recommended due to its flexibility, consistency, and future-proofing.

However, if you want to extract all the characters from a given index to the end of the string, you can use `slice()` without the end argument:

let str = "Hello, World!";

str.slice(7); // Preferred

str.substr(7, str.length - 7); // Less ideal