one_pass_interpreter.js

const code1 = '((7 1 +) 5 +) ((8 ((8 ((8 3 +) 1 +) +) 1 +) +) 1 +) +';
const code2 = '4 3 -';
const code3 = '(2 3 +) (23 21 +) +';

const operators = {
    '+': (args) => args[0] + args[1],
    '-': (args) => args[0] - args[1],
    '*': (args) => args[0] * args[1],
    '/': (args) => args[0] / args[1],
};

let counter = 0;

const parse = (source) => {
    console.log(counter++, `${' '.repeat(counter*4)}${source}`);
    const re = /\(.*?\)/;
    
    let hasNestedScope = re.test(source);
    
    while (hasNestedScope) {
        let nestedCounter = 0;
        
        const parentesisIndexes = [0, 0];
        
        for(let i = 0; i < source.length; i++) {
            const char = source[i];
            if(char === '(') {
                nestedCounter++;
                if(nestedCounter === 1) parentesisIndexes[0] = i+1;
            }
            if(char === ')') {
                nestedCounter--;
                if(nestedCounter === 0) parentesisIndexes[1] = i;
            }
        }
        const scopeCode = source.slice(...parentesisIndexes);
        const scopeResult = parse(scopeCode);
        
        source = source.replace(`(${scopeCode})`, scopeResult);
        
        hasNestedScope = re.test(source);
    }

    const tokens = source.split(' ');
    
    let stack = [];
    
    for (const token of tokens) {
        if (token in operators) break;

        stack.push(Number(token));
    }
    
    const op = tokens.pop();
    
    const result = operators[op](stack);
    
    return result;
};

console.log( 'Result:', parse(code1), '\n' );
counter = 0;
console.log( 'Result:', parse(code2), '\n' );
counter = 0;
console.log( 'Result:', parse(code3), '\n' );