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What are the primitive data types in JavaScript?
JavaScript values are either primitives or objects. Primitive values are immutable: an operation can produce a new value, but it does not change the primitive value itself. Objects are a separate language type and can hold properties.
Here are the seven primitive types and their common uses:
string: text, written inside quotes, such as"hello". Strings are immutable.number: ordinary integer and decimal values, such as42and3.14.NaNandInfinityare also Number values;typeof NaNis"number".bigint: whole integers that can be larger than Number’s safe integer range, written with annsuffix, such as42n, or created withBigInt(). BigInt does not represent fractions.boolean: one of the two logical values,trueorfalse, commonly used in conditions.undefined: the single valueundefined, commonly seen when a value was not supplied or found, a variable was declared without an initial value, or a function returns no value.null: the single valuenull, used to represent an explicit null value.symbol: a unique, immutable value, often used as a non-colliding key on an object.
This example declares one value of each type:
const name = "Ada"; // string
const score = 42; // number
const huge = 9007199254740993n; // bigint
const isReady = true; // boolean
let notSet; // undefined
const empty = null; // null
const id = Symbol("id"); // symbolA variable is not permanently locked to its first type. For example, a variable declared with let can hold a number and later a string:
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let value = 42;
value = "forty-two";How do you identify a primitive with typeof?
The MDN Web Docs typeof reference describes the operator as returning a string that indicates the type of the operand’s value. For the primitive examples above, its results are:
typeof "Ada"; // "string"
typeof 42; // "number"
typeof 42n; // "bigint"
typeof true; // "boolean"
typeof undefined; // "undefined"
typeof Symbol(); // "symbol"
typeof null; // "object" — historical exceptiontypeof null returns "object" for historical compatibility, not because null is an object. The behavior dates to JavaScript’s first implementation, when an old type-tag representation gave null the tag used for objects. To check for null, use strict equality:
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if (value === null) {
// value is null
}typeof is not a complete object-classification tool: arrays also return "object". Use Array.isArray(value) when you need to determine whether a value is an array.
What is the difference between undefined and null?
Both values relate to absence, but they communicate different things. undefined commonly means a value was not supplied or has not been found—for example, a declared but uninitialized variable. null is an explicit null value, often assigned when code deliberately represents an empty or absent value.
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let notSet; // undefined
const deliberatelyEmpty = null;When should you use number or bigint?
Use number for ordinary integer and decimal arithmetic. JavaScript Number values use the IEEE 754 double-precision 64-bit binary format. Its safe integer bounds are −(2^53 − 1) through 2^53 − 1, as described in MDN’s JavaScript data types and data structures guide. Integers beyond those bounds may not be represented exactly as Number values.
Use bigint when you need exact integer arithmetic beyond Number’s safe range. BigInt can represent arbitrarily large integers, but not fractional values. Number and BigInt arithmetic operands cannot be mixed directly; convert explicitly when an operation needs a common type.
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const count = 9007199254740993n;
const nextCount = count + 1n; // BigInt arithmetic
// 1n + 1; // TypeError: cannot mix BigInt and NumberWhy can primitive values have methods?
Strings, numbers, and booleans can be used with methods because JavaScript automatically wraps a primitive temporarily when accessing its properties. For example, a string primitive can call toUpperCase():
"hello".toUpperCase(); // "HELLO"This does not make the primitive an object. Explicit wrapper objects such as new String("hi"), new Number(2), and new Boolean(false) are objects, unlike their primitive counterparts. Avoid these constructors in ordinary beginner code; use primitive literals instead.
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What beginner mistakes should you watch for?
- Assuming
typeofalways tells you whether something is an object:typeof nullandtypeof []both return"object". Usevalue === nullfor null checks andArray.isArray(value)for arrays. - Mixing Number and BigInt: arithmetic with both types requires an explicit conversion. BigInt is integer-only; Number handles ordinary integer and fractional values.
- Overlooking implicit conversion: JavaScript converts values in some operations. For example,
42 + "1"produces the string"421", not the number43. Check the types of values when an operation gives an unexpected result. - Confusing a primitive with a wrapper object: a literal such as
2is a Number primitive;new Number(2)creates an object.
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