How do you convert a string to a number?

Four common options, and they disagree on the interesting inputs.

Number("42");        // 42
parseInt("42", 10);  // 42
parseFloat("42.5");  // 42.5
+"42";               // 42

Where they differ

Number() is strict: the whole string must be a valid number, or you get NaN. The parse* functions are lenient — they read from the start and stop at the first character that does not fit, which is why "42px" becomes 42.

Pick by intent

Use Number() (or +) when the input should be a clean number and anything else is an error — form fields, API payloads. Use parseInt/parseFloat when you are deliberately extracting a number from text, such as "16px" from a computed style.

Always pass the radix to parseInt

parseInt("08");       // 8 today, but relies on default behaviour
parseInt("08", 10);   // 8, explicitly decimal
parseInt("ff", 16);   // 255

Validate the result

NaN propagates silently through arithmetic, so check before using the value:

const n = Number(input);
if (Number.isNaN(n)) return;            // reject
if (!Number.isFinite(n)) return;        // also rejects Infinity

Two traps worth remembering: Number("") is 0, not NaN, so an empty field can silently become zero. And integers beyond Number.MAX_SAFE_INTEGER lose precision — use BigInt("9007199254740993") for identifiers and large counts.