snippetjavascriptTip

Capitalize the first letter of a JavaScript string

Submitted by: @import:30-seconds-of-code·Mar 2, 2026·

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javascriptletterfirstthecapitalizestring

Problem

As software engineers, we are often tasked with transforming data into presentable information. A common part of this process is capitalizing the first letter of a string.
In order to accomplish this task, we can use array destructuring in combination with String.prototype.toUpperCase() to capitalize the first letter of the string. We can then use Array.prototype.join() to combine the capitalized first with the ...rest of the characters. Additionally, we might want to transform the rest of the string into lowercase, using String.prototype.toLowerCase() and a boolean argument.
Conversely, we can also decapitalize the first letter of a string using the same technique, but using String.prototype.toLowerCase() instead.

Solution

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

capitalize('fooBar'); // 'FooBar'
capitalize('fooBar', true); // 'Foobar'

Conversely, we can also decapitalize the first letter of a string using the same technique, but using String.prototype.toLowerCase() instead.

Code Snippets

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

capitalize('fooBar'); // 'FooBar'
capitalize('fooBar', true); // 'Foobar'

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

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

Context

From 30-seconds-of-code: capitalize-first-letter-of-string

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