How to get a variable type in Typescript

Question

I have a variable   abc number string    How can I check its type  I want to do something like below   if  abc type      number            do something

User · Answer

I have checked a variable if it is a boolean or not as below  console log isBoolean this myVariable      Similarly we have   isNumber this myVariable   isString this myvariable     and so on

User · Answer

I d like to add that TypeGuards only work on strings or numbers  if you want to compare an object use instanceof  if task id instanceof UUID        foo

User · Answer

I suspect you can adjust your approach a little and use something along the lines of the example here  https   www typescriptlang org docs handbook advanced-types html user-defined-type-guards function isFish pet  Fish   Bird   pet is Fish     return  pet as Fish  swim     undefined

User · Answer

For     abc number string    Use the JavaScript operator typeof    if  typeof abc      number            do something     TypeScript understands typeof   This is called a typeguard    More  For classes you would use instanceof e g   class Foo    class Bar         Later if  fooOrBar instanceof Foo        TypeScript now knows that  fooOrBar  is  Foo      There are also other type guards e g  in etc  https   basarat gitbooks io typescript content docs types typeGuard html

User · Answer

The other answers are right  but when you re dealing with interfaces you cannot use typeof or instanceof because interfaces don t get compiled to javascript   Instead you can use a typecast   function check typeguard to check your variable   interface Car       drive    void      honkTheHorn    void     interface Bike       drive    void      ringTheBell    void     function start vehicle  Bike   Car         vehicle drive            typecast and check if the function exists     if    lt Bike gt vehicle  ringTheBell            const bike     lt Bike gt vehicle           bike ringTheBell          else           const car     lt Car gt vehicle           car honkTheHorn              And this is the compiled JavaScript in ES2017   function start vehicle        vehicle drive        if  vehicle ringTheBell            const bike   vehicle          bike ringTheBell              else           const car   vehicle          car honkTheHorn

User · Answer

Type guards in typescript To determine the type of a variable after a conditional statement you can use type guards  A type guard in typescript is the following   An expression which allows you to narrow down the type of something within a conditional block   In other words it is an expression within a conditional block from where the typescript compiler has enough information to narrow down the type  The type will be more specific within the block of the type guard because the compiler has inferred more information about the type  Example declare let abc  number   string      typeof abc      string  is a type guard if  typeof abc      string            abc  string     console log  abc is a string here     else          abc  number  only option because the previous type guard removed the option of string     console log  abc is a number here      Besides the typeof operator there are built in type guards like instanceof  in and even your own type guards

[typescript] How to get a variable type in Typescript?

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