Learn JavaScript type coercion with plus, minus, comparisons, equality, null, undefined, strings, booleans, and tricky conversion examples.

JavaScript Type Conversions and Coercion Rules

JavaScript converts values automatically in many operators. The confusing part is that not every operator converts the same way. Use these snippets to learn the rules by output, not by memorizing a table.

For mixed expressions, also keep operator precedence in mind.

Plus

+ performs string concatenation if either side becomes a string. Otherwise it performs numeric addition.

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

- is numeric only, so both sides are converted to numbers.

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"); // 0

Comparisons

Relational comparisons usually convert mixed types to numbers, except string-to-string comparisons, which compare lexicographically.

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); // true

Equality

Strict equality (===) does not coerce values. Loose equality (==) does, which is why examples with null, undefined, false, 0, and "" need extra care.

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([] === []); // false

Mixed 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); // -2

Type Conversion and Coercion