Transform a string from camel case to snake case and vice versa

Question

I would like transform string with uppercase to string with underscore like :

 - "blablaBlabla" to "blabla_blabla" 
 - "firstName" to "first_name"

And conversely :

 - "blabla_blabla" to "blablaBlabla"
 - "first_Name" to "firstName"

I use Typescript, but I think for that, there is no difference with the Javascript.

Thank's in advance.

Jérémy.

[This question](https://stackoverflow.com/questions/30521224/javascript-convert-pascalcase-to-underscore-case) asks for the opposite operation, but the process will be the same : using regex match `_` and the following letter which you put into a capturing group, extract the content of this capturing group, translate to uppercase, replace the match by the uppercase. — Aaron, May 06 '19 at 12:27

adiga · Answer 1 · 2019-05-06T13:31:17.673

You could get all the upper case letters with [A-Z] and replace the match with _ + m.toLowerCase()

To change it the other way, match all _([a-z]) to get the alphabet to a capturing group. And then use toUpperCase on the capture

function trasnform1(str) {
  return str.replace(/[A-Z]/g, (m) => '_' + m.toLowerCase())
}

function trasnform2(str) {
  return str.replace(/_([a-z])/g, (m, p1) => p1.toUpperCase())
}

console.log(trasnform1("blablaBlabla"))
console.log(trasnform1("firstName"))

console.log(trasnform2("blabla_blabla"))
console.log(trasnform2("first_name"))

score 2 · Accepted Answer · edited May 06 '19 at 13:29

let word = "firstName";
let output = "";

// for conversion
for (let i = 0; i < word.length; i++) {
  if (word[i] === word[i].toUpperCase()) {
    output += "_" + word[i].toLowerCase();
  } else {
    output += word[i];
  }
}
console.log(output);

let source = output;
output = "";

//for reversion
for (let i = 0; i < source.length; i++) {
  if (source[i] === "_") {
    i++;
    output += source[i].toUpperCase();
  } else {
    output += source[i];
  }
}
console.log(output);

score 0 · Answer 3 · answered Jul 23 '21 at 06:03

// Camel to snake and snake to camel case
function changeCasing(input) {
  if (!input) {
    return '';
  }
  if (input.indexOf('_') > -1) {
    const regex = new RegExp('_.', 'gm');
    return input.replace(regex, (match) => {
      const char = match.replace("_", "");
      return char.toUpperCase();
    });
  } else {
    const regex = new RegExp('[A-Z]', 'gm');
    return input.replace(regex, (match) => {
      return `_${match.toLowerCase()}`;
    });
  }
}

score 0 · Answer 4 · answered Apr 10 '22 at 15:56

This is example written in TypeScript. It's more readable using method chaining.

export const snake2Camel = (snake: string): string => {
  return snake.split('').reduce((prev: string, cur: string) => {
    if (prev.includes("_")) {
      prev = prev.substring(0, prev.length - 1);
      cur = cur.toUpperCase();
    }
    return prev + cur;
  }, "");
};

Transform a string from camel case to snake case and vice versa

4 Answers4