How to multiply a number by 2 in JavaScript

The core operation is the multiplication operator *.
A function that takes a single argument x and returns x * 2 is sufficient for doubling any numeric value.

1. Classic function declaration

function multiplyByTwo(x) {
  return x * 2;
}

2. Arrow function (concise ES6 syntax)

const multiplyByTwo = x => x * 2;

3. Function expression

const multiplyByTwo = function(x) {
  return x * 2;
};

All three forms are equivalent; choose the style that matches your codebase.

Example usage

console.log(multiplyByTwo(5));   // → 10
console.log(multiplyByTwo(3.5)); // → 7
console.log(multiplyByTwo(-4));  // → -8

Why it works

• The * operator multiplies two numbers, coercing its operands to numeric values if necessary².
• JavaScript treats 2 as a numeric literal, so x * 2 directly yields the doubled value.

Edge considerations

• Non‑numeric inputs: JavaScript will attempt type coercion (multiplyByTwo("3") → 6). For stricter behavior, add validation.
• BigInt: The operator also works with BigInt values², e.g., multiplyByTwo(10n) → 20n.

Naming variations

Different tutorials use different function names (multiplyByTwo, double, etc.). The functionality remains identical; only the identifier changes.

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References

• Multiplication operator documentation²
• Common examples of numeric multiplication in JavaScript¹³