Type declarations
You can add type declarations to variables, function parameters and function return types. The
type is written after a colon following the variable name, like this: var num: number = 5; The
compiler will then check the types (where possible) during compilation and report type errors.

var num: number = 5;
num = "this is a string"; // error: Type 'string' is not assignable to type 'number'.

The basic types are :
number (both integers and floating point numbers)
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string
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boolean
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Array. You can specify the types of an array's elements. There are two equivalent ways to
define array types: Array<T> and T[]. For example:
number[] - array of numbers
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Array<string> - array of strings
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Tuples. Tuples have a fixed number of elements with specific types.
[boolean, string] - tuple where the first element is a boolean and the second is a
string.
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[number, number, number] - tuple of three numbers.
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{} - object, you can define its properties or indexer
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{name: string, age: number} - object with name and age attributes
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{[key: string]: number} - a dictionary of numbers indexed by string
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enum - { Red = 0, Blue, Green } - enumeration mapped to numbers
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Function. You specify types for the parameters and return value:
(param: number) => string - function taking one number parameter returning string
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() => number - function with no parameters returning an number.
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(a: string, b?: boolean) => void - function taking a string and optionally a boolean with
no return value.
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any - Permits any type. Expressions involving any are not type checked.
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void - represents "nothing", can be used as a function return value. Only null and undefined
are part of the void type.
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never
let foo: never; -As the type of variables under type guards that are never true.
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function error(message: string): never { throw new Error(message); } - As the return
type of functions that never return.
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null - type for the value null. null is implicitly part of every type, unless strict null checks are
enabled.