create unit test for string concatenation in javascript

To create a unit test for string concatenation in Javascript, we can use a testing framework like Jest.

Assuming that we have a function that takes two strings and concatenates them:

index.tsx
function concatenateStrings(str1, str2) {
  return str1 + str2;
}
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We can create a unit test to verify its functionality:

index.tsx
test('concatenateStrings function should concatenate two strings', () => {
  const str1 = 'Hello';
  const str2 = 'World';
  const result = concatenateStrings(str1, str2);

  expect(result).toBe('HelloWorld');
});
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In the above test, we define two strings to concatenate (str1 and str2), call the concatenateStrings function with the two strings as arguments, and then use the expect function to assert that the result is equal to the expected concatenated string ('HelloWorld').

This simple example follows the principles of Test-Driven Development (TDD) in which we write tests before writing any code, and continue writing tests that are meant to initially fail, and then write code to make them pass.