Building a Calculator
Learn how to build a state-driven calculation engine with clean number formatting and operator management.
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📋 Copy Calculator State Machine Engine Snippet
// Pure State-Machine Calculator Architecture
class CalculatorEngine {
constructor() {
this.current = "0";
this.previous = null;
this.operation = null;
}
appendNumber(num) {
if (this.current === "0" && num !== ".") this.current = "";
if (num === "." && this.current.includes(".")) return;
this.current += num;
}
chooseOperation(op) {
if (this.current === "") return;
if (this.previous !== null) this.compute();
this.operation = op;
this.previous = this.current;
this.current = "";
}
}⚠️ 5 Fatal Traps & JavaScript Pitfalls
Trap #1: Using eval() to Compute Calculations
Evaluating mathematical expressions with
eval(expressionString) allows arbitrary malicious JavaScript execution and code injection. Build an explicit parser or state machine.Trap #2: IEEE 754 Floating-Point Arithmetic Drift
Performing
0.1 + 0.2 returns 0.30000000000000004. Round results to a reasonable precision (e.g. parseFloat(result.toFixed(10))).Trap #3: Division by Zero Returning Infinity
In JavaScript,
5 / 0 evaluates to Infinity rather than an error. Check for division by zero and display an explicit error state.Trap #4: Multiple Decimal Points in a Single Operand
Failing to guard against repeated decimal button presses allows invalid inputs like
3.1.4.5.Trap #5: Operator Chaining State Bugs
Pressing
5 + * 3 without properly updating the active operation leads to unexpected NaN results.💬 Frequently Asked Questions
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