String API: string.includes()

string.includes() determines if a string can be found inside another one

finding a single character

• can be done (a character is also a string, in JS)
• reports false if character was not found

find a string

• that matches exactly

search for an empty string, is always true

• in an empty string
• in abc

special/corner cases

• search for undefined in a string fails
• searches are case-sensitive
• must NOT be a regular expression

coerces the searched "thing" into a string

• e.g. from a number
• e.g. from an array
• e.g. from an object, with a toString() method

takes a position from where to start searching

• does not find a after position 1 in abc
• even the position gets coerced

invalid positions get converted to 0

• e.g. undefined
• negative numbers
• NaN

Links

spec The official specification, actually quite good to read for this function.
mdn docs The Mozilla Developer Network docs, contains good examples.