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# AI solution for JavaScript Coding Question on BFE.dev 124. calculate arithmetic expression

One possible solution is:

``/** * @param {string} str * @return {number} */function calculate(str) {  const tokens = tokenize(str);  function parseExpr() {    let left = parseTerm();    while (tokens.length > 0 && (tokens[0] === '+' || tokens[0] === '-')) {      const op = tokens.shift();      const right = parseTerm();      if (op === '+') {        left += right;      } else {        left -= right;      }    }    return left;  }  function parseTerm() {    let left = parseFactor();    while (tokens.length > 0 && (tokens[0] === '*' || tokens[0] === '/')) {      const op = tokens.shift();      const right = parseFactor();      if (op === '*') {        left *= right;      } else {        left /= right;      }    }    return left;  }  function parseFactor() {    if (tokens[0] === '(') {      tokens.shift(); // consume '('      const value = parseExpr();      tokens.shift(); // consume ')'      return value;    } else {      return Number(tokens.shift());    }  }  // start parsing from the expression level  return parseExpr();}``

The `calculate` function first tokenizes the input string using the `tokenize` function from the previous question. Then it defines three helper functions: `parseExpr` for parsing addition and subtraction expressions, `parseTerm` for parsing multiplication and division expressions, and `parseFactor` for parsing numbers and parentheses.

Each function uses a recursive descent parsing technique, meaning that it calls other functions to parse sub-expressions when needed. The parsing order is determined by operator precedence and associativity rules, which are encoded in the function structure.

The result of the `calculate` function is the value of the top-level expression, which is obtained by calling `parseExpr`.