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String API: string.includes()

Finds string within another string.

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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


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