What is the difference between == and === in JavaScript?

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Both operators compare two values, but == performs loose equality with type coercion, while === performs strict equality without any coercion. === is the safer default because its behaviour is predictable and easy to reason about. Strict equality (===) === returns true only when both operands have the same type and the same value . No conversion takes place. 1 === 1 // true 1 === "1" // false null === undefined // false true === 1 // false Loose equality (==) == attempts to convert the operands to a common type using the abstract equality algorithm before comparing. This is what produces many of the surprising results that gave JavaScript its reputation for quirky equality. 1 == "1" // true (string coerced to number) true == 1 // true (boolean coerced to number) 0 == false // true null == undefined // true (special case) null == 0 // false (another special case) [] == false // true ([] → "" → 0) [1] == 1 // true Coercion rules in brief If one side is a number and the other a string , the string is converted to a number. If either side is a boolean , it is converted to a number ( true → 1 , false → 0 ). If one side is an object , it is converted to a primitive via valueOf() or toString() . null and undefined are loosely equal to each other but to nothing else. NaN is never equal to anything, including itself — both NaN == NaN and NaN === NaN return false . Object.is Object.is is nearly identical to === but fixes two edge cases: Object.is(NaN, NaN) is true , and Object.is(+0, -0) is false . When to use which Use === and !== by default. The only widely accepted use of == is value == null , which conveniently checks for both null and undefined at once. All major style guides (Airbnb, Google, ESLint's eqeqeq ) enforce strict equality. Always use ===. Know the coercion surprises: '' == 0, null == undefined, [] == false. The value == null shorthand for null/undefined is the one acceptable == use. NaN !== NaN is a bonus gotcha.