Пример минималистичного парсера

parser.js

function parse(text) {
    const state = { text, pos: 0 }
    const tokens = []
    while (state.pos < text.length) {
        const token = word(state) || delimiter(state)
        if (!token) {
            throw new Error(`Unknown character at ${state.pos}`)
        }
        tokens.push(token)
    }
    return tokens
}

function word(state) {
    const start = state.pos
    while (state.pos < state.text.length) {
        if (!isWordChar(state.text.charCodeAt(state.pos))) {
            break
        }
        state.pos++
    }

    if (start !== state.pos) {
        return {
            type: 'word',
            value: state.text.slice(start, state.pos)
        }
    }
}

function delimiter(state) {
    const ch = state.text.charCodeAt(state.pos)
    if (ch === 32 /* пробел */ || ch === 44 /* , */) {
        return {
            type: 'delimiter',
            value: state.text[state.pos++]
        }
    }
}

function isWordChar(ch) {
    return ch >= 97 && ch <= 122 /* a-z */
}

console.log(parse('hello world'))
// [
//   { type: 'word', value: 'hello' },
//   { type: 'delimiter', value: ' ' },
//   { type: 'word', value: 'world' }
// ]