JavaScript Type Conversions and Coercion Rules
JavaScript automatically converts values between types in many situations — a behavior known as type coercion. This guide helps you master how operators
like +, -, comparisons (<, >, ==, ===), and more behave when mixing strings, numbers, booleans, null, and undefined.
plus
Concatenates strings
plus
const log = (value) => console.log(value);
log(null + 1); // 1
log(undefined + 1); // NaN
log(false + 1); // 1
log(true + 1); // 2
log("" + 1); // "1"
log("s" + 1); // "s1"
log("1" + 1); // "11"
log(1 + null); // 1
log(1 + undefined); // NaN
log(1 + false); // 1
log(1 + true); // 2
log(1 + ""); // "1"
log(1 + "s"); // "1s"
log(1 + "1"); // "11"minus
Behaves like other operators, converts to numbers
minus
const log = (value) => console.log(value);
log(null - 1); // -1
log(undefined - 1); // NaN
log(false - 1); // -1
log(true - 1); // 0
log("" - 1); // -1
log("s" - 1); // NaN
log("1" - 1); // 0
log(1 - null); // 1
log(1 - undefined); // NaN
log(1 - false); // 1
log(1 - true); // 0
log(1 - ""); // 1
log(1 - "s"); // NaN
log(1 - "1"); // 0comparisons
When comparing values of different types, they convert to numbers
comparisons
const log = (value) => console.log(value);
log(null < 1); // true
log(undefined < 1); // false
log(undefined > 1); // false
log(false < 1); // true
log(true > 0); // true
log("" > 0); // false
log("s" > 1); // false
log("1" > 0); // true
log("12" > "111"); // true
log("12" > 111); // false
log("break");
log(null <= 1); // true
log(null <= 0); // true
log(undefined <= 1); // false
log(undefined >= 1); // false
log(false <= 1); // true
log(true >= 0); // true
log("" >= 0); // true
log("s" >= 1); // false
log("1" >= 0); // true
log("12" >= "111"); // true
log("12" >= 111); // false
log("break");
log("0" >= 0); // trueequality
Strict equality === doesn’t convert values
== and ===
const log = (value) => console.log(value);
log(null == null); // true
log(undefined == undefined); // true
log(undefined == null); // true
log(null == undefined); // true
log(0 == false); // true
log(1 == true); // true
log(0 == ""); // true
log(1 == "s"); // false
log(1 == "1"); // true
// [] converted to ''
log(0 == []); // true
log(0 == {}); // false
log("break");
log(null === null); // true
log(undefined === undefined); // true
log(undefined === null); // false
log([] === []); // falserandom conversions
random conversions
const log = (value) => console.log(value);
log("10" + 2 * "5"); // 1010
log("" + 0 + 1); // "01"
log("" + 0 - 1); // -1
log("" - 1 + 0); // -1
log(true + false); // 1
log(6 / "3"); // 2
log("2" * "3"); // 6
log(4 + 5 + "px"); // "9px"
log("$" + 4 + 5); // "$45"
log("4" - 2); // 2
log("4px" - 2); // NaN
log(" -9 " + 5); // " -9 5"
log(" -9 " - 5); // -14
log(null + 1); // 1
log(undefined + 1); // NaN
log(" \t \n" - 2); // -2Last updated on