parseInt() vs toString() in JavaScript — Explained Like You're 5

If you’ve ever needed to convert a string to a number, or a number to a string, you’ve likely run into parseInt() and toString().
They look simple, but they come with a secret weapon you need to understand: radix.

Let’s unpack all of it — clearly and practically.

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🤔 First of All... What Is Radix?

Think of radix as the base of a number system.

You already use radix 10 every day — that’s the decimal system (0 to 9).
Computers often use other bases too:

So when you call parseInt('1010', 2), you’re saying:

“Hey JavaScript, this string '1010' is in binary, please convert it to a decimal number.”

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🧠 parseInt() — Turning Strings into Integers

🧾 Syntax:

parseInt(string, radix);

• string → The input you want to convert (like '42', '1010', 'ff')

• radix → The base of the number system you're using (like 2, 10, 16)

✅ Examples:

parseInt('42', 10);     // 42  ← Simple decimal
parseInt('1010', 2);    // 10  ← Binary to decimal
parseInt('1A', 16);     // 26  ← Hexadecimal to decimal
parseInt('100px', 10);  // 100 ← Stops at first non-digit
parseInt('abc', 10);    // NaN ← Not a number

⚠️ Important: Always include the radix

This is good:

parseInt('08', 10);  // ✅ Output: 8

This is risky:

parseInt('08');      // ❌ Might output 8 or 0 depending on the browser

Older JavaScript engines sometimes treated strings like '08' as octal (base 8), which led to bugs. Be safe — always specify the radix.

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🧠 toString() — Turning Numbers into Strings

🧾 Syntax:

number.toString(radix);

• number → Any number you want to convert

• radix → (Optional) Which base you want the result in — default is 10.

✅ Examples:

(42).toString();       // "42"
(255).toString(16);    // "ff" ← Decimal to hex
(8).toString(2);       // "1000" ← Decimal to binary

🔍 Why (42).toString() and not 42.toString()?

Because 42.toString() is parsed as a float and gives an error.
Use parentheses or a space like this:

(42).toString(2);  // ✅
42..toString(2);   // ✅ (two dots)
42 .toString(2);   // ✅ (with space)

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🧪 Side-by-Side Breakdown

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👀 Real-world Use Cases

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🧠 Gotchas to Watch Out For

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🎯 Final Summary

// Convert string to number
parseInt("1010", 2);   // → 10

// Convert number to string
(255).toString(16);    // → "ff"

🧠 Golden Rule:

Use parseInt() when reading a number from a string.
Use toString() when outputting a number as text (especially in binary, hex, etc.).