JavaScript Type Conversion

Coercion is JavaScript converting types behind your back. Conversion is you doing it on purpose, with a function that makes your intent obvious to anyone reading the code. This lesson is the complete toolkit: strings to numbers, numbers to strings, dates, booleans, and the full table of what every value turns into.

// Strings to Numbers: Number()

Number() converts the whole string or nothing. Spaces around the number are allowed; anything else makes it NaN:

console.log(Number("3.14"), Number(Math.PI));
console.log(Number("  42  "), Number(""), Number(" "));
console.log(Number("0x1F"), Number("1e3"), Number("-5"));
console.log(Number("99 88"), Number("John"), Number("12px"));

// parseInt() and parseFloat()

These read from the start of the string and stop at the first character that does not fit. parseInt also drops any decimals:

console.log(parseInt("42px"), parseFloat("3.14abc"));
console.log(parseInt("3.99"), parseInt("  7 days"));
console.log(parseInt("px42"), parseInt(""));        // must START with a number
console.log(parseInt("101", 2), parseInt("ff", 16)); // optional base

// The Unary + Operator

A + in front of a value converts it exactly like Number(). If it cannot, the result is still a number: NaN.

const y = "5";
const x = +y;
console.log(x, typeof x);

const z = +"John";
console.log(z, typeof z);

// Numbers to Strings

String() works on any number: a literal, a variable, or an expression. The number method toString() does the same (JS toString()):

const n = 123;
console.log(String(n), String(100 + 23), n.toString(), (100 + 23).toString());

The formatting methods from Number Methods also return strings, rounded the way you choose:

// Dates to Numbers, and Back

Inside, a date is a number: milliseconds since January 1, 1970 (UTC). Number(), getTime(), and unary + all give it to you, and new Date(ms) turns it back:

const d = new Date(Date.UTC(2026, 0, 1));

console.log(Number(d), d.getTime(), +d);
console.log(new Date(0).toISOString());
console.log(new Date(1767225600000).toISOString());

String(new Date()) and new Date().toString() give the long local text you saw in JS toString(). An invalid date converts to NaN: Number(new Date("nope")).

// Date Parts to Strings

The get methods return numbers. Convert and pad them to build your own format, and remember months start at 0:

const d = new Date(2026, 0, 5, 14, 7);

console.log(d.getMonth(), (d.getMonth() + 1).toString());
console.log(d.getMinutes(), String(d.getMinutes()).padStart(2, "0"));
console.log(d.toLocaleString("en-US", { month: "long" }));

// Booleans to Numbers and Strings, and Back

console.log(Number(false), Number(true));
console.log(String(false), true.toString());
console.log(Boolean(1), Boolean(0), Boolean("0"), Boolean(""));

// BigInts and Symbols

console.log(BigInt("12"), Number(12n), String(10n ** 20n));
console.log(BigInt(1.5));

A BigInt has no fractions, so a decimal cannot become one. A Symbol cannot become a number at all: Number(Symbol("x")) is TypeError: Cannot convert a Symbol value to a number.

// The Conversion Table

What every common value becomes with Number(), String(), and Boolean(). The rows marked * are the ones that surprise people:

Two rules explain most of the table. An array converts through its string: [20] is "20", which is the number 20, but [10,20] is "10,20", which is not a number. And null becomes 0 while undefined becomes NaN.

// Quick Check

console.log(Number(""), parseInt(""), Number("12px"), parseInt("12px"));

// Try It

▶ Open Playground

1. Click Open Playground and run it: each input converted with Number, parseInt, and parseFloat. Explain each difference
2. Get the current time in milliseconds three different ways
3. Build a "YYYY-MM-DD" string from a date using get methods, String, and padStart
4. Pick five rows of the conversion table and check them yourself