Creating a wave of string in Javascript

Question

I can't seem to figure it out how to make a wave from a string in Javascript.

Rules:

1. The input will always be lower case string.
2. Ignore whitespace.

Expected result:

wave("hello") => ["Hello", "hEllo", "heLlo", "helLo", "hellO"]

wave (" h e y ") => [" H e y ", " h E y ", " h e Y "]

wave ("") => []

This is as far as I got. Current code will give me an answer ["hello", "hello", "hello", "hello", "hello"]. I'm thinking using second for loop and somehow capitalize each new letter but I'am stumped. Also I would appreciate if answer would avoid using loop inside loop O(n^2). Because of BIG O Scalability.

const wave = (str) => {
    if(typeof str === 'string' && str === str.toLowerCase()){
       for (let index = 0; index < str.length; index++) {
        array.push(str);
    }
    for (let index = 0; index < str.length; index++) {
            console.log(array);   
    }
    }else{
        alert(`${str} is either not a string or not lowercase`);
    }
}
wave("hello");

Answer 1

You could take an outer loop for visiting the characters and if a non space character is found, create a new string with an uppercase letter at this position.

function wave(string) {
    var result = [],
        i;

    for (i = 0; i < string.length; i++) {
        if (string[i] === ' ') continue;
        result.push(Array.from(string, (c, j) => i === j ? c.toUpperCase() : c).join(''));
    }
    return result;
}

console.log(wave("hello"));   // ["Hello", "hEllo", "heLlo", "helLo", "hellO"]
console.log(wave(" h e y ")); // [" H e y ", " h E y ", " h e Y "]
console.log(wave(""));        // []

.as-console-wrapper { max-height: 100% !important; top: 0; }

Answer 2

This does it. However, if you have spaces in your string, it will output string without any "waved letter" (since also space is handled):

const wave = (str = '') => {
  return str
    .split('')
    .map((letter, i, arr) => `${arr.slice(0, i)}${letter.toUpperCase()}${arr.slice(i + 1, arr.length)}`.replace(/,/g, ''));
}

console.log(wave('wave'));
console.log(wave('foo bar'));
console.log(wave());

Answer 3

You can take each char in string in for loop and make it uppercase and the append with prefix and post fix string

var array=[];
const wave = (str) => {
    if(typeof str === 'string' && str === str.toLowerCase()){
       for (let index = 0; index < str.length; index++) {
       if (str[index] === ' ') continue;
       waveChar=str.charAt(index).toUpperCase();
       preStr=str.substring(0,index);
       postStr=str.substring(index,str.length-1);
        array.push(preStr+waveChar+postStr);
    }
    
    }else{
        alert(`${str} is either not a string or not lowercase`);
    }
}
wave("hello");
console.log(array);