JSFiddle - React, Tailwind, and code Playground

JavaScript

let string = "JavaScript is Awesome!",
		output = '';

for( let i = 0; i < string.length; i++ ) {
	if( string[ i ] === 'e' ) output += '3';
  else if( string[ i ] === 'o' ) output += '0';
  else if( string[ i ] === 'a' ) output += '4';
  else if( string[ i ] === 'i' ) output += '1';
	else output += string[ i ];
}

alert( output ); // J4v4Scr1pt 1s Aw3s0m3!