Substring

A string has 10 characters, but we wantonly a part of this string—the first 5, the last 3. With substring we get a string from within an existing string.

In Node.js, substring receives 2 arguments: the first index of the substring we are trying to take, and the last index. The second argument is not a count.

First example

Let us begin with this example. We have an input string "cat123." To get the "cat" part, we use a first index of 0, and a last index of 3.

var value = "cat123";
console.log("VALUE: " + value);

// Take a substring of the first 3 characters.
// ... Start at index 0.
// ... Continue until index 3.
var result = value.substring(0, 3);
console.log("SUBSTRING: " + result);
VALUE: cat123
SUBSTRING: cat

Substring, 1 argument

Substring can be used with just 1 argument. This is the start index. We can think of this argument as the number of characters we want to skip over.

var value = "x_frog";

// Skip over first 2 characters.
var result = value.substring(2);

console.log("VALUE:     " + value);
console.log("SUBSTRING: " + result);
VALUE:     x_frog
SUBSTRING: frog

With substring, we have a way to extract parts of strings. For optimal performance, using the original string and not changing it at all is best, but this is not always possible.