como usar o decapitalize no javascript es6

Decapitalizes the first letter of a string.

Use array destructuring and String.toLowerCase() to decapitalize first letter, ...rest to get array of characters after first letter and then Array.prototype.join('') to make it a string again. Omit the upperRest parameter to keep the rest of the string intact, or set it to true to convert to uppercase.

const decapitalize = ([first, ...rest], upperRest = false) =>
  first.toLowerCase() + (upperRest ? rest.join('').toUpperCase() : rest.join(''));

decapitalize('FooBar'); // 'fooBar'
decapitalize('FooBar', true); // 'fOOBAR'

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