“Type ‘string | string[]’ is not assignable to type ‘string’

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The problem in your code is, that params.username can be either a string or a string[].

The following scenario returns a simple string

const {query} = url.parse('example.com?test=1', true);
// query.test === '1'

while the example below returns an array because the same query-parameter key is used twice

const {query} = url.parse('example.com?test=1&test=2', true);
//query.test === ['1', '2']

So you could handle the array case explicitly e.g.

const username = Array.isArray(params.username) ? params.username[0] : params.username

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