You can capitalize the first letter of a string in JavaScript by using some simple string and array functions. Here’s the complete code sample, followed by an explanation of how it works:

const greeting = "hello"
const capitalize = string => [
  ...string.slice(0, 1).toUpperCase(),
  ...string.slice(1)
].join("")

// Here's the same function, rewritten in long-form JavaScript
const capitalize = function(string) {
  return [
    ...string.slice(0, 1).toUpperCase(),
    ...string.slice(1)
  ].join("")
}

capitalize(greeting) // "Hello"

How it works — slices and spreads

This works by using slice to split the string by characters into two sections: the first character—using .slice(0, 1)—and the remainder of the string, with .slice(1). Without a second argument provided to slice, it will grab the rest of the string.

Strings can be coerced into arrays in JavaScript using the ES6 spread operator. By spreading the string into two sections, and calling .toUpperCase() on the first character, we can effectively capitalize the first letter of the string, and then join it back together inside of an array of characters. For instance:

const greeting = "hello"
const capitalizeWithoutJoin = string => [  
  ...string.slice(0, 1).toUpperCase(),
  ...string.slice(1)
]

capitalizeWithoutJoin(greeting) // ["H", "e", "l", "l", "o"]

The spread operator takes both strings, and combines the array of characters from each into a single array, which is returned by the function. The original capitalize function takes that array of characters, and joins them back into a string.

Capitalize multiple words in a sentence

The advantage of capitalizing the first letter of a string in this way is that you can re-use it for multiple words. For instance, if you want to capitalize the first letter of every word in a larger string, you can map through the words and call capitalize on each:

// const capitalize = string ...
const long_string = "hello world how is it going"
const capitalized = long_string.split(" ").map(string => capitalize(string)).join(" ")  

capitalized // "Hello World How Is It Going"