includes() JavaScript String Method – How to Check If a String Includes a Substring
Whenever you use includes() on a string, the method does the following:
- It checks if the calling string includes the method's first argument.
- It returns
true
if the string includes the argument. Otherwise, it returnsfalse
.
note
- A calling string is a string on which you used
includes()
. So, in"Hello, world!".includes("world")
,"Hello, world!"
is the calling string. includes()
is sometimes written asString.prototype.includes()
because it is a method of theString
object'sprototype
property.
Syntax of the includes()
Method
includes()
accepts two arguments. Here is the syntax:
callingString.includes(valueToFind, startIndex);
Argument 1: valueToFind
A valueToFind
is the first argument accepted by the includes()
method. It defines the value you wish to find in the calling string.
Example 1: Check if a given string includes "day"
"SunDay, Tuesday, and Friday are good Days".includes("day");
// The invocation above will return: true
Note that the includes()
method is case-sensitive.
Example 2: Check if a given string includes "DAY"
"SunDay, Tuesday, and Friday are good Days".includes("DAY");
// The invocation above will return: false
Example 3: Check if a given string includes 3
"Day1, day-3, and day 6 are good Days".includes(3);
// The invocation above will return: true
Argument 2: startIndex
The startIndex
argument is optional. It specifies the index position where you want the computer to start searching for the valueToFind
argument.
note
The startIndex
argument's default value is 0
. Therefore, if omitted, the search will begin at index 0
.
Example 1: From the 13th index position, check if a given string includes "day"
"SunDay, Tuesday, and Friday are good Days".includes("day", 13);
// The invocation above will return: true
Example 2: From the 1st index position, check if a given string includes "Sun"
"SunDay, Tuesday, and Friday are good Days".includes("Sun", 1);
// The invocation above will return: false
tip
- To find the index position of a substring within a string, use
indexOf()
. - You can also use
includes()
on an array.
Overview
includes()
checks if its calling string includes the method's first argument.