Parses the 'expr' rule in the BNF grammar.

parse_expr.js

ParseExpr() {
    // expr ::= mulexpr { addop mulexpr }
    let expr = this.ParseMulExpr();
    let optoken;
    while (optoken = this.NextTokenIs(['+', '-'])) {
        const right = this.ParseMulExpr();
        if (optoken.text === '+') {
            expr = new Expression_Add(optoken, expr, right);
        } else {
            expr = new Expression_Subtract(optoken, expr, right);
        }
    }
    return expr;
}