Data Types
JavaScript values are one of several built-in types.
Syntax
let text = 'hi';
let num = 10;
let flag = true;JavaScript has several built-in data types. The most common are:
- String — text, in quotes:
'hello'. - Number — any number:
42,3.14. - Boolean —
trueorfalse. - Array — an ordered list:
[1, 2, 3]. - Object — key/value pairs:
{ name: 'Sam' }.
There are also null, undefined, and others.
Example
Loading editor…
Press Run to execute the code.
When to use it
- Check whether a value returned from an API is a string or a number before processing it.
- Guard against passing the wrong type to a function that expects a boolean flag.
- Choose the correct data structure (array vs object) when modeling a list of user records.
More examples
Common primitive types
Declares variables covering the five most common JavaScript primitive types and logs their typeof results.
const name = "Alice"; // string
const age = 30; // number
const active = true; // boolean
const score = null; // null
let address; // undefined
console.log(typeof name, typeof age, typeof active);Objects and arrays are reference types
Shows that both plain objects and arrays report "object" for typeof, so Array.isArray is needed to distinguish them.
const user = { id: 1, name: "Bob" }; // object
const tags = ["js", "web"]; // array (also an object)
console.log(typeof user); // "object"
console.log(Array.isArray(tags)); // trueDynamic typing in action
Demonstrates JavaScript's dynamic typing: the same variable can hold different types at different times.
let value = 42;
console.log(typeof value); // "number"
value = "hello";
console.log(typeof value); // "string"
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