Detect a space in a string

I am writing a lexer for a programming language.

I have managed to detect all characters but spaces (not general whitespaces, just spaces).

I’ve tried just using letter == " " and letter === " ".
I’ve also looked at some SO question that recommend using some Regex, but that also didn’t work.

Relevant code snippet (this is part of a bigger Lexer class):

generateOutput(input) {
        const wordList = input.split(' ');
        const keyword = wordList[0];
        const identifier = wordList[1];

        const output = [
            "[KEYWORD] " + keyword.toUpperCase(),
            "[IDENTIFIER] " + identifier
        ]

        for (let i = 2; i < wordList.length; i++) {
            const word = wordList[i];
            const words = word.split("");
            words.forEach((letter) => {
                if (upperCase.includes(letter)) {
                    output.push("[KEY] U_" + letter)
                } else if (lowerCase.includes(letter)) {
                    output.push("[KEY] L_" + letter)
                } else if (symbols.includes(letter)) {
                    output.push("[SYMBOL] " + letter)
                } else if (letter === " ") {
                    output.push("[KEY] SPACE")
                }
            })
        }

        return output
    }

>Solution :

You used input.split(' '), like I said in my comment. That deleted all of the spaces.

var a = "this is a sentence"
console.log( a.split( " " ) );
//["this","is","a","sentence"] <- contains absolutely no spaces