Chai.js - Check if string contains substring from a list

expect(thatSelectedItem).to.contain.oneOf(['jpg', 'png', 'gif'])

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This lands with v4.3.0 (docs). oneOf can be chained with contain, contains, include and includes, which will work with both arrays and strings. It's strongly recommended to use because the error messages you get with it are much cleaner.

With plain Chai (no additional plugins), you can use match:

expect(thatSelectedItem).to.match(/(?:jpg|png|gif)/)