Chapter 1: Types in JavaScript

ECMA Language Types

ECMA language types are:

1. undefined
2. null
3. Boolean
4. string
5. Number
6. Object

Built-in Types in JavaScript

JavaScript built-in types are:

• null
• undefined
• boolean
• number
• string
• object
• Symbol (added in ES6)

Note: Except object, all are called primitive types.

Whatever value you will see in JS, if you write typeof before that, you will get these types.

Examples:

typeof Symbol(1) // "symbol"

typeof null == "object" // true (Bug!)

Bug: It is not fixed because there are so much web context exist and rely on this bug. Fixing this will create more bugs, so they left it as it is.

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typeof function foo(){} // "function"
typeof [1,2] // "object"

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Functions in JavaScript

In JS, there is no built-in type as function but it is a subtype of object.

Function is nothing but a callable object.

Internally, it uses [[Call]] property to invoke function.

Note: Each function has length property attached to it that is equal to the number of params.

Example:

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function foo(a, b, c) { }
console.log(foo.length) // 3

Same goes with Array!

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Values as Types

In JavaScript, variables don’t have types, values have types.

Variables can hold any value at any time.

Note: typeof always returns a string.

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undefined vs undeclared

undefined:

• A variable that has been declared in the accessible scope but at the moment has no other value in it.

Example:

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var a;
console.log(typeof a); // "undefined"

undeclared:

• If a variable is not declared (not defined yet), and you try to use it, you get Reference Error!

Example:

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var b = 42;
console.log(typeof c); // "undefined"

console.log(b = c); // ReferenceError!

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Type of undeclared

If you use typeof a where a is not declared yet, you will always get "undefined" without error.

Learn about DEBUG keyword in JS!